bpo-42663: Fix parsing TZ strings in zoneinfo module (GH-23825)

zipinfo now supports the full range of values in the TZ string
determined by RFC 8536 and detects all invalid formats.
Both Python and C implementations now raise exceptions of the same
type on invalid data.

4 files changed, 326 insertions, 258 deletions

diff --git a/Lib/test/test_zoneinfo/test_zoneinfo.py b/Lib/test/test_zoneinfo/test_zoneinfo.py
index ae921f7..3766cea 100644
--- a/Lib/test/test_zoneinfo/test_zoneinfo.py
+++ b/Lib/test/test_zoneinfo/test_zoneinfo.py
@@ -1001,6 +1001,80 @@ class TZStrTest(ZoneInfoTestBase):
 
                 self.assertEqual(dt_act, dt_utc)
 
+    def test_extreme_tzstr(self):
+        tzstrs = [
+            # Extreme offset hour
+            "AAA24",
+            "AAA+24",
+            "AAA-24",
+            "AAA24BBB,J60/2,J300/2",
+            "AAA+24BBB,J60/2,J300/2",
+            "AAA-24BBB,J60/2,J300/2",
+            "AAA4BBB24,J60/2,J300/2",
+            "AAA4BBB+24,J60/2,J300/2",
+            "AAA4BBB-24,J60/2,J300/2",
+            # Extreme offset minutes
+            "AAA4:00BBB,J60/2,J300/2",
+            "AAA4:59BBB,J60/2,J300/2",
+            "AAA4BBB5:00,J60/2,J300/2",
+            "AAA4BBB5:59,J60/2,J300/2",
+            # Extreme offset seconds
+            "AAA4:00:00BBB,J60/2,J300/2",
+            "AAA4:00:59BBB,J60/2,J300/2",
+            "AAA4BBB5:00:00,J60/2,J300/2",
+            "AAA4BBB5:00:59,J60/2,J300/2",
+            # Extreme total offset
+            "AAA24:59:59BBB5,J60/2,J300/2",
+            "AAA-24:59:59BBB5,J60/2,J300/2",
+            "AAA4BBB24:59:59,J60/2,J300/2",
+            "AAA4BBB-24:59:59,J60/2,J300/2",
+            # Extreme months
+            "AAA4BBB,M12.1.1/2,M1.1.1/2",
+            "AAA4BBB,M1.1.1/2,M12.1.1/2",
+            # Extreme weeks
+            "AAA4BBB,M1.5.1/2,M1.1.1/2",
+            "AAA4BBB,M1.1.1/2,M1.5.1/2",
+            # Extreme weekday
+            "AAA4BBB,M1.1.6/2,M2.1.1/2",
+            "AAA4BBB,M1.1.1/2,M2.1.6/2",
+            # Extreme numeric offset
+            "AAA4BBB,0/2,20/2",
+            "AAA4BBB,0/2,0/14",
+            "AAA4BBB,20/2,365/2",
+            "AAA4BBB,365/2,365/14",
+            # Extreme julian offset
+            "AAA4BBB,J1/2,J20/2",
+            "AAA4BBB,J1/2,J1/14",
+            "AAA4BBB,J20/2,J365/2",
+            "AAA4BBB,J365/2,J365/14",
+            # Extreme transition hour
+            "AAA4BBB,J60/167,J300/2",
+            "AAA4BBB,J60/+167,J300/2",
+            "AAA4BBB,J60/-167,J300/2",
+            "AAA4BBB,J60/2,J300/167",
+            "AAA4BBB,J60/2,J300/+167",
+            "AAA4BBB,J60/2,J300/-167",
+            # Extreme transition minutes
+            "AAA4BBB,J60/2:00,J300/2",
+            "AAA4BBB,J60/2:59,J300/2",
+            "AAA4BBB,J60/2,J300/2:00",
+            "AAA4BBB,J60/2,J300/2:59",
+            # Extreme transition seconds
+            "AAA4BBB,J60/2:00:00,J300/2",
+            "AAA4BBB,J60/2:00:59,J300/2",
+            "AAA4BBB,J60/2,J300/2:00:00",
+            "AAA4BBB,J60/2,J300/2:00:59",
+            # Extreme total transition time
+            "AAA4BBB,J60/167:59:59,J300/2",
+            "AAA4BBB,J60/-167:59:59,J300/2",
+            "AAA4BBB,J60/2,J300/167:59:59",
+            "AAA4BBB,J60/2,J300/-167:59:59",
+        ]
+
+        for tzstr in tzstrs:
+            with self.subTest(tzstr=tzstr):
+                self.zone_from_tzstr(tzstr)
+
     def test_invalid_tzstr(self):
         invalid_tzstrs = [
             "PST8PDT",  # DST but no transition specified
@@ -1008,16 +1082,33 @@ class TZStrTest(ZoneInfoTestBase):
             "GMT,M3.2.0/2,M11.1.0/3",  # Transition rule but no DST
             "GMT0+11,M3.2.0/2,M11.1.0/3",  # Unquoted alphanumeric in DST
             "PST8PDT,M3.2.0/2",  # Only one transition rule
-            # Invalid offsets
-            "STD+25",
-            "STD-25",
-            "STD+374",
-            "STD+374DST,M3.2.0/2,M11.1.0/3",
-            "STD+23DST+25,M3.2.0/2,M11.1.0/3",
-            "STD-23DST-25,M3.2.0/2,M11.1.0/3",
+            # Invalid offset hours
+            "AAA168",
+            "AAA+168",
+            "AAA-168",
+            "AAA168BBB,J60/2,J300/2",
+            "AAA+168BBB,J60/2,J300/2",
+            "AAA-168BBB,J60/2,J300/2",
+            "AAA4BBB168,J60/2,J300/2",
+            "AAA4BBB+168,J60/2,J300/2",
+            "AAA4BBB-168,J60/2,J300/2",
+            # Invalid offset minutes
+            "AAA4:0BBB,J60/2,J300/2",
+            "AAA4:100BBB,J60/2,J300/2",
+            "AAA4BBB5:0,J60/2,J300/2",
+            "AAA4BBB5:100,J60/2,J300/2",
+            # Invalid offset seconds
+            "AAA4:00:0BBB,J60/2,J300/2",
+            "AAA4:00:100BBB,J60/2,J300/2",
+            "AAA4BBB5:00:0,J60/2,J300/2",
+            "AAA4BBB5:00:100,J60/2,J300/2",
             # Completely invalid dates
             "AAA4BBB,M1443339,M11.1.0/3",
             "AAA4BBB,M3.2.0/2,0349309483959c",
+            "AAA4BBB,,J300/2",
+            "AAA4BBB,z,J300/2",
+            "AAA4BBB,J60/2,",
+            "AAA4BBB,J60/2,z",
             # Invalid months
             "AAA4BBB,M13.1.1/2,M1.1.1/2",
             "AAA4BBB,M1.1.1/2,M13.1.1/2",
@@ -1037,6 +1128,26 @@ class TZStrTest(ZoneInfoTestBase):
             # Invalid julian offset
             "AAA4BBB,J0/2,J20/2",
             "AAA4BBB,J20/2,J366/2",
+            # Invalid transition time
+            "AAA4BBB,J60/2/3,J300/2",
+            "AAA4BBB,J60/2,J300/2/3",
+            # Invalid transition hour
+            "AAA4BBB,J60/168,J300/2",
+            "AAA4BBB,J60/+168,J300/2",
+            "AAA4BBB,J60/-168,J300/2",
+            "AAA4BBB,J60/2,J300/168",
+            "AAA4BBB,J60/2,J300/+168",
+            "AAA4BBB,J60/2,J300/-168",
+            # Invalid transition minutes
+            "AAA4BBB,J60/2:0,J300/2",
+            "AAA4BBB,J60/2:100,J300/2",
+            "AAA4BBB,J60/2,J300/2:0",
+            "AAA4BBB,J60/2,J300/2:100",
+            # Invalid transition seconds
+            "AAA4BBB,J60/2:00:0,J300/2",
+            "AAA4BBB,J60/2:00:100,J300/2",
+            "AAA4BBB,J60/2,J300/2:00:0",
+            "AAA4BBB,J60/2,J300/2:00:100",
         ]
 
         for invalid_tzstr in invalid_tzstrs:
diff --git a/Lib/zoneinfo/_zoneinfo.py b/Lib/zoneinfo/_zoneinfo.py
index eede15b..b77dc0e 100644
--- a/Lib/zoneinfo/_zoneinfo.py
+++ b/Lib/zoneinfo/_zoneinfo.py
@@ -517,8 +517,8 @@ class _DayOffset:
     __slots__ = ["d", "julian", "hour", "minute", "second"]
 
     def __init__(self, d, julian, hour=2, minute=0, second=0):
-        if not (0 + julian) <= d <= 365:
-            min_day = 0 + julian
+        min_day = 0 + julian  # convert bool to int
+        if not min_day <= d <= 365:
             raise ValueError(f"d must be in [{min_day}, 365], not: {d}")
 
         self.d = d
@@ -560,11 +560,11 @@ class _CalendarOffset:
     )
 
     def __init__(self, m, w, d, hour=2, minute=0, second=0):
-        if not 0 < m <= 12:
-            raise ValueError("m must be in (0, 12]")
+        if not 1 <= m <= 12:
+            raise ValueError("m must be in [1, 12]")
 
-        if not 0 < w <= 5:
-            raise ValueError("w must be in (0, 5]")
+        if not 1 <= w <= 5:
+            raise ValueError("w must be in [1, 5]")
 
         if not 0 <= d <= 6:
             raise ValueError("d must be in [0, 6]")
@@ -634,18 +634,21 @@ def _parse_tz_str(tz_str):
 
     offset_str, *start_end_str = tz_str.split(",", 1)
 
-    # fmt: off
     parser_re = re.compile(
-        r"(?P<std>[^<0-9:.+-]+|<[a-zA-Z0-9+\-]+>)" +
-        r"((?P<stdoff>[+-]?\d{1,2}(:\d{2}(:\d{2})?)?)" +
-            r"((?P<dst>[^0-9:.+-]+|<[a-zA-Z0-9+\-]+>)" +
-                r"((?P<dstoff>[+-]?\d{1,2}(:\d{2}(:\d{2})?)?))?" +
-            r")?" + # dst
-        r")?$" # stdoff
+        r"""
+        (?P<std>[^<0-9:.+-]+|<[a-zA-Z0-9+-]+>)
+        (?:
+            (?P<stdoff>[+-]?\d{1,3}(?::\d{2}(?::\d{2})?)?)
+            (?:
+                (?P<dst>[^0-9:.+-]+|<[a-zA-Z0-9+-]+>)
+                (?P<dstoff>[+-]?\d{1,3}(?::\d{2}(?::\d{2})?)?)?
+            )? # dst
+        )? # stdoff
+        """,
+        re.ASCII|re.VERBOSE
     )
-    # fmt: on
 
-    m = parser_re.match(offset_str)
+    m = parser_re.fullmatch(offset_str)
 
     if m is None:
         raise ValueError(f"{tz_str} is not a valid TZ string")
@@ -696,16 +699,17 @@ def _parse_tz_str(tz_str):
 
 
 def _parse_dst_start_end(dststr):
-    date, *time = dststr.split("/")
-    if date[0] == "M":
+    date, *time = dststr.split("/", 1)
+    type = date[:1]
+    if type == "M":
         n_is_julian = False
-        m = re.match(r"M(\d{1,2})\.(\d).(\d)$", date)
+        m = re.fullmatch(r"M(\d{1,2})\.(\d).(\d)", date, re.ASCII)
         if m is None:
             raise ValueError(f"Invalid dst start/end date: {dststr}")
         date_offset = tuple(map(int, m.groups()))
         offset = _CalendarOffset(*date_offset)
     else:
-        if date[0] == "J":
+        if type == "J":
             n_is_julian = True
             date = date[1:]
         else:
@@ -715,38 +719,54 @@ def _parse_dst_start_end(dststr):
         offset = _DayOffset(doy, n_is_julian)
 
     if time:
-        time_components = list(map(int, time[0].split(":")))
-        n_components = len(time_components)
-        if n_components < 3:
-            time_components.extend([0] * (3 - n_components))
-        offset.hour, offset.minute, offset.second = time_components
+        offset.hour, offset.minute, offset.second = _parse_transition_time(time[0])
 
     return offset
 
 
+def _parse_transition_time(time_str):
+    match = re.fullmatch(
+        r"(?P<sign>[+-])?(?P<h>\d{1,3})(:(?P<m>\d{2})(:(?P<s>\d{2}))?)?",
+        time_str,
+        re.ASCII
+    )
+    if match is None:
+        raise ValueError(f"Invalid time: {time_str}")
+
+    h, m, s = (int(v or 0) for v in match.group("h", "m", "s"))
+
+    if h > 167:
+        raise ValueError(
+            f"Hour must be in [0, 167]: {time_str}"
+        )
+
+    if match.group("sign") == "-":
+        h, m, s = -h, -m, -s
+
+    return h, m, s
+
+
 def _parse_tz_delta(tz_delta):
-    match = re.match(
-        r"(?P<sign>[+-])?(?P<h>\d{1,2})(:(?P<m>\d{2})(:(?P<s>\d{2}))?)?",
+    match = re.fullmatch(
+        r"(?P<sign>[+-])?(?P<h>\d{1,3})(:(?P<m>\d{2})(:(?P<s>\d{2}))?)?",
         tz_delta,
+        re.ASCII
     )
     # Anything passed to this function should already have hit an equivalent
     # regular expression to find the section to parse.
     assert match is not None, tz_delta
 
-    h, m, s = (
-        int(v) if v is not None else 0
-        for v in map(match.group, ("h", "m", "s"))
-    )
+    h, m, s = (int(v or 0) for v in match.group("h", "m", "s"))
 
     total = h * 3600 + m * 60 + s
 
-    if not -86400 < total < 86400:
+    if h > 24:
         raise ValueError(
-            f"Offset must be strictly between -24h and +24h: {tz_delta}"
+            f"Offset hours must be in [0, 24]: {tz_delta}"
         )
 
     # Yes, +5 maps to an offset of -5h
     if match.group("sign") != "-":
-        total *= -1
+        total = -total
 
     return total
diff --git a/Misc/NEWS.d/next/Library/2022-05-06-15-49-57.gh-issue-86826.rf006W.rst b/Misc/NEWS.d/next/Library/2022-05-06-15-49-57.gh-issue-86826.rf006W.rst
new file mode 100644
index 0000000..02cd75e
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2022-05-06-15-49-57.gh-issue-86826.rf006W.rst
@@ -0,0 +1,4 @@
+:mod:`zipinfo` now supports the full range of values in the TZ string
+determined by RFC 8536 and detects all invalid formats.
+Both Python and C implementations now raise exceptions of the same
+type on invalid data.
diff --git a/Modules/_zoneinfo.c b/Modules/_zoneinfo.c
index eb4e522..3f14dd1 100644
--- a/Modules/_zoneinfo.c
+++ b/Modules/_zoneinfo.c
@@ -61,21 +61,21 @@ struct TransitionRuleType {
 
 typedef struct {
     TransitionRuleType base;
-    uint8_t month;
-    uint8_t week;
-    uint8_t day;
-    int8_t hour;
-    int8_t minute;
-    int8_t second;
+    uint8_t month;      /* 1 - 12 */
+    uint8_t week;       /* 1 - 5 */
+    uint8_t day;        /* 0 - 6 */
+    int16_t hour;       /* -167 - 167, RFC 8536 §3.3.1 */
+    int8_t minute;      /* signed 2 digits */
+    int8_t second;      /* signed 2 digits */
 } CalendarRule;
 
 typedef struct {
     TransitionRuleType base;
-    uint8_t julian;
-    unsigned int day;
-    int8_t hour;
-    int8_t minute;
-    int8_t second;
+    uint8_t julian;     /* 0, 1 */
+    uint16_t day;       /* 0 - 365 */
+    int16_t hour;       /* -167 - 167, RFC 8536 §3.3.1 */
+    int8_t minute;      /* signed 2 digits */
+    int8_t second;      /* signed 2 digits */
 } DayRule;
 
 struct StrongCacheNode {
@@ -133,15 +133,14 @@ ts_to_local(size_t *trans_idx, int64_t *trans_utc, long *utcoff,
 static int
 parse_tz_str(zoneinfo_state *state, PyObject *tz_str_obj, _tzrule *out);
 
-static Py_ssize_t
-parse_abbr(const char *const p, PyObject **abbr);
-static Py_ssize_t
-parse_tz_delta(const char *const p, long *total_seconds);
-static Py_ssize_t
-parse_transition_time(const char *const p, int8_t *hour, int8_t *minute,
-                      int8_t *second);
-static Py_ssize_t
-parse_transition_rule(const char *const p, TransitionRuleType **out);
+static int
+parse_abbr(const char **p, PyObject **abbr);
+static int
+parse_tz_delta(const char **p, long *total_seconds);
+static int
+parse_transition_time(const char **p, int *hour, int *minute, int *second);
+static int
+parse_transition_rule(const char **p, TransitionRuleType **out);
 
 static _ttinfo *
 find_tzrule_ttinfo(_tzrule *rule, int64_t ts, unsigned char fold, int year);
@@ -1327,14 +1326,14 @@ calendarrule_year_to_timestamp(TransitionRuleType *base_self, int year)
     }
 
     int64_t ordinal = ymd_to_ord(year, self->month, month_day) - EPOCHORDINAL;
-    return ((ordinal * 86400) + (int64_t)(self->hour * 3600) +
-            (int64_t)(self->minute * 60) + (int64_t)(self->second));
+    return ordinal * 86400 + (int64_t)self->hour * 3600 +
+            (int64_t)self->minute * 60 + self->second;
 }
 
 /* Constructor for CalendarRule. */
 int
-calendarrule_new(uint8_t month, uint8_t week, uint8_t day, int8_t hour,
-                 int8_t minute, int8_t second, CalendarRule *out)
+calendarrule_new(int month, int week, int day, int hour,
+                 int minute, int second, CalendarRule *out)
 {
     // These bounds come from the POSIX standard, which describes an Mm.n.d
     // rule as:
@@ -1343,33 +1342,36 @@ calendarrule_new(uint8_t month, uint8_t week, uint8_t day, int8_t hour,
     //   5, 1 <= m <= 12, where week 5 means "the last d day in month m" which
     //   may occur in either the fourth or the fifth week). Week 1 is the first
     //   week in which the d'th day occurs. Day zero is Sunday.
-    if (month <= 0 || month > 12) {
-        PyErr_Format(PyExc_ValueError, "Month must be in (0, 12]");
+    if (month < 1 || month > 12) {
+        PyErr_Format(PyExc_ValueError, "Month must be in [1, 12]");
         return -1;
     }
 
-    if (week <= 0 || week > 5) {
-        PyErr_Format(PyExc_ValueError, "Week must be in (0, 5]");
+    if (week < 1 || week > 5) {
+        PyErr_Format(PyExc_ValueError, "Week must be in [1, 5]");
         return -1;
     }
 
-    // If the 'day' parameter type is changed to a signed type,
-    // "day < 0" check must be added.
-    if (/* day < 0 || */ day > 6) {
+    if (day < 0 || day > 6) {
         PyErr_Format(PyExc_ValueError, "Day must be in [0, 6]");
         return -1;
     }
 
+    if (hour < -167 || hour > 167) {
+        PyErr_Format(PyExc_ValueError, "Hour must be in [0, 167]");
+        return -1;
+    }
+
     TransitionRuleType base = {&calendarrule_year_to_timestamp};
 
     CalendarRule new_offset = {
         .base = base,
-        .month = month,
-        .week = week,
-        .day = day,
-        .hour = hour,
-        .minute = minute,
-        .second = second,
+        .month = (uint8_t)month,
+        .week = (uint8_t)week,
+        .day = (uint8_t)day,
+        .hour = (int16_t)hour,
+        .minute = (int8_t)minute,
+        .second = (int8_t)second,
     };
 
     *out = new_offset;
@@ -1409,40 +1411,45 @@ dayrule_year_to_timestamp(TransitionRuleType *base_self, int year)
     // always transitions on a given calendar day (other than February 29th),
     // you would use a Julian day, e.g. J91 always refers to April 1st and J365
     // always refers to December 31st.
-    unsigned int day = self->day;
+    uint16_t day = self->day;
     if (self->julian && day >= 59 && is_leap_year(year)) {
         day += 1;
     }
 
-    return ((days_before_year + day) * 86400) + (self->hour * 3600) +
-           (self->minute * 60) + self->second;
+    return (days_before_year + day) * 86400 + (int64_t)self->hour * 3600 +
+           (int64_t)self->minute * 60 + self->second;
 }
 
 /* Constructor for DayRule. */
 static int
-dayrule_new(uint8_t julian, unsigned int day, int8_t hour, int8_t minute,
-            int8_t second, DayRule *out)
+dayrule_new(int julian, int day, int hour, int minute,
+            int second, DayRule *out)
 {
     // The POSIX standard specifies that Julian days must be in the range (1 <=
     // n <= 365) and that non-Julian (they call it "0-based Julian") days must
     // be in the range (0 <= n <= 365).
     if (day < julian || day > 365) {
-        PyErr_Format(PyExc_ValueError, "day must be in [%u, 365], not: %u",
+        PyErr_Format(PyExc_ValueError, "day must be in [%d, 365], not: %d",
                      julian, day);
         return -1;
     }
 
+    if (hour < -167 || hour > 167) {
+        PyErr_Format(PyExc_ValueError, "Hour must be in [0, 167]");
+        return -1;
+    }
+
     TransitionRuleType base = {
         &dayrule_year_to_timestamp,
     };
 
     DayRule tmp = {
         .base = base,
-        .julian = julian,
-        .day = day,
-        .hour = hour,
-        .minute = minute,
-        .second = second,
+        .julian = (uint8_t)julian,
+        .day = (int16_t)day,
+        .hour = (int16_t)hour,
+        .minute = (int8_t)minute,
+        .second = (int8_t)second,
     };
 
     *out = tmp;
@@ -1599,21 +1606,18 @@ parse_tz_str(zoneinfo_state *state, PyObject *tz_str_obj, _tzrule *out)
     const char *p = tz_str;
 
     // Read the `std` abbreviation, which must be at least 3 characters long.
-    Py_ssize_t num_chars = parse_abbr(p, &std_abbr);
-    if (num_chars < 1) {
-        PyErr_Format(PyExc_ValueError, "Invalid STD format in %R", tz_str_obj);
+    if (parse_abbr(&p, &std_abbr)) {
+        if (!PyErr_Occurred()) {
+            PyErr_Format(PyExc_ValueError, "Invalid STD format in %R", tz_str_obj);
+        }
         goto error;
     }
 
-    p += num_chars;
-
     // Now read the STD offset, which is required
-    num_chars = parse_tz_delta(p, &std_offset);
-    if (num_chars < 0) {
+    if (parse_tz_delta(&p, &std_offset)) {
         PyErr_Format(PyExc_ValueError, "Invalid STD offset in %R", tz_str_obj);
         goto error;
     }
-    p += num_chars;
 
     // If the string ends here, there is no DST, otherwise we must parse the
     // DST abbreviation and start and end dates and times.
@@ -1621,12 +1625,12 @@ parse_tz_str(zoneinfo_state *state, PyObject *tz_str_obj, _tzrule *out)
         goto complete;
     }
 
-    num_chars = parse_abbr(p, &dst_abbr);
-    if (num_chars < 1) {
-        PyErr_Format(PyExc_ValueError, "Invalid DST format in %R", tz_str_obj);
+    if (parse_abbr(&p, &dst_abbr)) {
+        if (!PyErr_Occurred()) {
+            PyErr_Format(PyExc_ValueError, "Invalid DST format in %R", tz_str_obj);
+        }
         goto error;
     }
-    p += num_chars;
 
     if (*p == ',') {
         // From the POSIX standard:
@@ -1636,14 +1640,11 @@ parse_tz_str(zoneinfo_state *state, PyObject *tz_str_obj, _tzrule *out)
         dst_offset = std_offset + 3600;
     }
     else {
-        num_chars = parse_tz_delta(p, &dst_offset);
-        if (num_chars < 0) {
+        if (parse_tz_delta(&p, &dst_offset)) {
             PyErr_Format(PyExc_ValueError, "Invalid DST offset in %R",
                          tz_str_obj);
             goto error;
         }
-
-        p += num_chars;
     }
 
     TransitionRuleType **transitions[2] = {&start, &end};
@@ -1656,14 +1657,12 @@ parse_tz_str(zoneinfo_state *state, PyObject *tz_str_obj, _tzrule *out)
         }
         p++;
 
-        num_chars = parse_transition_rule(p, transitions[i]);
-        if (num_chars < 0) {
+        if (parse_transition_rule(&p, transitions[i])) {
             PyErr_Format(PyExc_ValueError,
                          "Malformed transition rule in TZ string: %R",
                          tz_str_obj);
             goto error;
         }
-        p += num_chars;
     }
 
     if (*p != '\0') {
@@ -1698,21 +1697,25 @@ error:
 }
 
 static int
-parse_uint(const char *const p, uint8_t *value)
+parse_digits(const char **p, int min, int max, int *value)
 {
-    if (!Py_ISDIGIT(*p)) {
-        return -1;
+    assert(max <= 3);
+    *value = 0;
+    for (int i = 0; i < max; i++, (*p)++) {
+        if (!Py_ISDIGIT(**p)) {
+            return (i < min) ? -1 : 0;
+        }
+        *value *= 10;
+        *value += (**p) - '0';
     }
-
-    *value = (*p) - '0';
     return 0;
 }
 
 /* Parse the STD and DST abbreviations from a TZ string. */
-static Py_ssize_t
-parse_abbr(const char *const p, PyObject **abbr)
+static int
+parse_abbr(const char **p, PyObject **abbr)
 {
-    const char *ptr = p;
+    const char *ptr = *p;
     const char *str_start;
     const char *str_end;
 
@@ -1741,7 +1744,7 @@ parse_abbr(const char *const p, PyObject **abbr)
         ptr++;
     }
     else {
-        str_start = p;
+        str_start = ptr;
         // From the POSIX standard:
         //
         //   In the unquoted form, all characters in these fields shall be
@@ -1751,6 +1754,9 @@ parse_abbr(const char *const p, PyObject **abbr)
             ptr++;
         }
         str_end = ptr;
+        if (str_end == str_start) {
+            return -1;
+        }
     }
 
     *abbr = PyUnicode_FromStringAndSize(str_start, str_end - str_start);
@@ -1758,12 +1764,13 @@ parse_abbr(const char *const p, PyObject **abbr)
         return -1;
     }
 
-    return ptr - p;
+    *p = ptr;
+    return 0;
 }
 
 /* Parse a UTC offset from a TZ str. */
-static Py_ssize_t
-parse_tz_delta(const char *const p, long *total_seconds)
+static int
+parse_tz_delta(const char **p, long *total_seconds)
 {
     // From the POSIX spec:
     //
@@ -1778,75 +1785,30 @@ parse_tz_delta(const char *const p, long *total_seconds)
     // The POSIX spec says that the values for `hour` must be between 0 and 24
     // hours, but RFC 8536 §3.3.1 specifies that the hours part of the
     // transition times may be signed and range from -167 to 167.
-    long sign = -1;
-    long hours = 0;
-    long minutes = 0;
-    long seconds = 0;
-
-    const char *ptr = p;
-    char buff = *ptr;
-    if (buff == '-' || buff == '+') {
-        // Negative numbers correspond to *positive* offsets, from the spec:
-        //
-        //   If preceded by a '-', the timezone shall be east of the Prime
-        //   Meridian; otherwise, it shall be west (which may be indicated by
-        //   an optional preceding '+' ).
-        if (buff == '-') {
-            sign = 1;
-        }
-
-        ptr++;
-    }
-
-    // The hour can be 1 or 2 numeric characters
-    for (size_t i = 0; i < 2; ++i) {
-        buff = *ptr;
-        if (!Py_ISDIGIT(buff)) {
-            if (i == 0) {
-                return -1;
-            }
-            else {
-                break;
-            }
-        }
+    int hours = 0;
+    int minutes = 0;
+    int seconds = 0;
 
-        hours *= 10;
-        hours += buff - '0';
-        ptr++;
-    }
-
-    if (hours > 24 || hours < 0) {
+    if (parse_transition_time(p, &hours, &minutes, &seconds)) {
         return -1;
     }
 
-    // Minutes and seconds always of the format ":dd"
-    long *outputs[2] = {&minutes, &seconds};
-    for (size_t i = 0; i < 2; ++i) {
-        if (*ptr != ':') {
-            goto complete;
-        }
-        ptr++;
-
-        for (size_t j = 0; j < 2; ++j) {
-            buff = *ptr;
-            if (!Py_ISDIGIT(buff)) {
-                return -1;
-            }
-            *(outputs[i]) *= 10;
-            *(outputs[i]) += buff - '0';
-            ptr++;
-        }
+    if (hours > 24 || hours < -24) {
+        return -1;
     }
 
-complete:
-    *total_seconds = sign * ((hours * 3600) + (minutes * 60) + seconds);
-
-    return ptr - p;
+    // Negative numbers correspond to *positive* offsets, from the spec:
+    //
+    //   If preceded by a '-', the timezone shall be east of the Prime
+    //   Meridian; otherwise, it shall be west (which may be indicated by
+    //   an optional preceding '+' ).
+    *total_seconds = -((hours * 3600L) + (minutes * 60) + seconds);
+    return 0;
 }
 
 /* Parse the date portion of a transition rule. */
-static Py_ssize_t
-parse_transition_rule(const char *const p, TransitionRuleType **out)
+static int
+parse_transition_rule(const char **p, TransitionRuleType **out)
 {
     // The full transition rule indicates when to change back and forth between
     // STD and DST, and has the form:
@@ -1858,10 +1820,10 @@ parse_transition_rule(const char *const p, TransitionRuleType **out)
     // does not include the ',' at the end of the first rule.
     //
     // The POSIX spec states that if *time* is not given, the default is 02:00.
-    const char *ptr = p;
-    int8_t hour = 2;
-    int8_t minute = 0;
-    int8_t second = 0;
+    const char *ptr = *p;
+    int hour = 2;
+    int minute = 0;
+    int second = 0;
 
     // Rules come in one of three flavors:
     //
@@ -1870,44 +1832,30 @@ parse_transition_rule(const char *const p, TransitionRuleType **out)
     //   3. Mm.n.d: Specifying by month, week and day-of-week.
 
     if (*ptr == 'M') {
-        uint8_t month, week, day;
+        int month, week, day;
         ptr++;
-        if (parse_uint(ptr, &month)) {
+
+        if (parse_digits(&ptr, 1, 2, &month)) {
             return -1;
         }
-        ptr++;
-        if (*ptr != '.') {
-            uint8_t tmp;
-            if (parse_uint(ptr, &tmp)) {
-                return -1;
-            }
-
-            month *= 10;
-            month += tmp;
-            ptr++;
+        if (*ptr++ != '.') {
+            return -1;
         }
-
-        uint8_t *values[2] = {&week, &day};
-        for (size_t i = 0; i < 2; ++i) {
-            if (*ptr != '.') {
-                return -1;
-            }
-            ptr++;
-
-            if (parse_uint(ptr, values[i])) {
-                return -1;
-            }
-            ptr++;
+        if (parse_digits(&ptr, 1, 1, &week)) {
+            return -1;
+        }
+        if (*ptr++ != '.') {
+            return -1;
+        }
+        if (parse_digits(&ptr, 1, 1, &day)) {
+            return -1;
         }
 
         if (*ptr == '/') {
             ptr++;
-            Py_ssize_t num_chars =
-                parse_transition_time(ptr, &hour, &minute, &second);
-            if (num_chars < 0) {
+            if (parse_transition_time(&ptr, &hour, &minute, &second)) {
                 return -1;
             }
-            ptr += num_chars;
         }
 
         CalendarRule *rv = PyMem_Calloc(1, sizeof(CalendarRule));
@@ -1923,33 +1871,22 @@ parse_transition_rule(const char *const p, TransitionRuleType **out)
         *out = (TransitionRuleType *)rv;
     }
     else {
-        uint8_t julian = 0;
-        unsigned int day = 0;
+        int julian = 0;
+        int day = 0;
         if (*ptr == 'J') {
             julian = 1;
             ptr++;
         }
 
-        for (size_t i = 0; i < 3; ++i) {
-            if (!Py_ISDIGIT(*ptr)) {
-                if (i == 0) {
-                    return -1;
-                }
-                break;
-            }
-            day *= 10;
-            day += (*ptr) - '0';
-            ptr++;
+        if (parse_digits(&ptr, 1, 3, &day)) {
+            return -1;
         }
 
         if (*ptr == '/') {
             ptr++;
-            Py_ssize_t num_chars =
-                parse_transition_time(ptr, &hour, &minute, &second);
-            if (num_chars < 0) {
+            if (parse_transition_time(&ptr, &hour, &minute, &second)) {
                 return -1;
             }
-            ptr += num_chars;
         }
 
         DayRule *rv = PyMem_Calloc(1, sizeof(DayRule));
@@ -1964,13 +1901,13 @@ parse_transition_rule(const char *const p, TransitionRuleType **out)
         *out = (TransitionRuleType *)rv;
     }
 
-    return ptr - p;
+    *p = ptr;
+    return 0;
 }
 
 /* Parse the time portion of a transition rule (e.g. following an /) */
-static Py_ssize_t
-parse_transition_time(const char *const p, int8_t *hour, int8_t *minute,
-                      int8_t *second)
+static int
+parse_transition_time(const char **p, int *hour, int *minute, int *second)
 {
     // From the spec:
     //
@@ -1982,12 +1919,9 @@ parse_transition_time(const char *const p, int8_t *hour, int8_t *minute,
     //   h[h][:mm[:ss]]
     //
     // RFC 8536 also allows transition times to be signed and to range from
-    // -167 to +167, but the current version only supports [0, 99].
-    //
-    // TODO: Support the full range of transition hours.
-    int8_t *components[3] = {hour, minute, second};
-    const char *ptr = p;
-    int8_t sign = 1;
+    // -167 to +167.
+    const char *ptr = *p;
+    int sign = 1;
 
     if (*ptr == '-' || *ptr == '+') {
         if (*ptr == '-') {
@@ -1996,32 +1930,31 @@ parse_transition_time(const char *const p, int8_t *hour, int8_t *minute,
         ptr++;
     }
 
-    for (size_t i = 0; i < 3; ++i) {
-        if (i > 0) {
-            if (*ptr != ':') {
-                break;
-            }
-            ptr++;
+    // The hour can be 1 to 3 numeric characters
+    if (parse_digits(&ptr, 1, 3, hour)) {
+        return -1;
+    }
+    *hour *= sign;
+
+    // Minutes and seconds always of the format ":dd"
+    if (*ptr == ':') {
+        ptr++;
+        if (parse_digits(&ptr, 2, 2, minute)) {
+            return -1;
         }
+        *minute *= sign;
 
-        uint8_t buff = 0;
-        for (size_t j = 0; j < 2; j++) {
-            if (!Py_ISDIGIT(*ptr)) {
-                if (i == 0 && j > 0) {
-                    break;
-                }
+        if (*ptr == ':') {
+            ptr++;
+            if (parse_digits(&ptr, 2, 2, second)) {
                 return -1;
             }
-
-            buff *= 10;
-            buff += (*ptr) - '0';
-            ptr++;
+            *second *= sign;
         }
-
-        *(components[i]) = sign * buff;
     }
 
-    return ptr - p;
+    *p = ptr;
+    return 0;
 }
 
 /* Constructor for a _tzrule.
@@ -2376,8 +2309,8 @@ get_local_timestamp(PyObject *dt, int64_t *local_ts)
         }
     }
 
-    *local_ts = (int64_t)(ord - EPOCHORDINAL) * 86400 +
-                (int64_t)(hour * 3600 + minute * 60 + second);
+    *local_ts = (int64_t)(ord - EPOCHORDINAL) * 86400L +
+                (int64_t)(hour * 3600L + minute * 60 + second);
 
     return 0;
 }