From f59a26a7c8e592e030fd55bc5e1dc02789dc7bbe Mon Sep 17 00:00:00 2001
From: "jan.nijtmans" <nijtmans@users.sourceforge.net>
Date: Thu, 14 Sep 2023 21:17:21 +0000
Subject: Fix [00655c867e]: ClockGetdatefieldsObjCmd(): avoid signed integer
 overflow and platform-dependent behavior

---
 generic/tclClock.c |  8 +++++---
 tests/clock.test   | 14 ++++++++++++++
 2 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/generic/tclClock.c b/generic/tclClock.c
index d379762..194cfb3 100644
--- a/generic/tclClock.c
+++ b/generic/tclClock.c
@@ -466,11 +466,13 @@ ClockGetdatefieldsObjCmd(
     }
 
     /*
-     * Extract Julian day.
+     * Extract Julian day. Always round the quotient down by subtracting 1
+     * when the remainder is negative (i.e. if the quotient was rounded up).
      */
 
-    fields.julianDay = (int) ((fields.localSeconds + JULIAN_SEC_POSIX_EPOCH)
-	    / SECONDS_PER_DAY);
+    fields.julianDay = (int) ((fields.localSeconds / SECONDS_PER_DAY) -
+	    ((fields.localSeconds % SECONDS_PER_DAY) < 0) +
+	    JULIAN_DAY_POSIX_EPOCH);
 
     /*
      * Convert to Julian or Gregorian calendar.
diff --git a/tests/clock.test b/tests/clock.test
index 587828f..4b12d5c 100644
--- a/tests/clock.test
+++ b/tests/clock.test
@@ -18600,6 +18600,20 @@ test clock-7.9 {Julian Day, two values} {
     clock scan {2440588 2440589} -format {%J %J} -gmt true
 } 86400
 
+test clock-7.10 {Julian Day, negative amount} {
+    # Note: %J does not accept negative input;
+    # add negative amounts to Julian day 0 instead
+    set s0 [clock scan 0 -format %J -gmt true]
+    set J0 [scan [clock format $s0 -format %J -gmt true] %lld]
+    set s0m1d [clock add $s0 -1 days]
+    set s0m24h [clock add $s0 -24 hours]
+    set J0m24h [scan [clock format $s0m24h -format %J -gmt true] %lld]
+    set s0m1s [clock add $s0 -1 seconds]
+    set J0m1s [scan [clock format $s0m1s -format %J -gmt true] %lld]
+    list $s0m1d $s0m24h $J0m24h $s0m1s $J0m1s $s0 $J0 \
+	[::tcl::mathop::== $s0m1d $s0m24h] [::tcl::mathop::== $J0m24h $J0m1s]
+} [list -210866889600 -210866889600 -1 -210866803201 -1 -210866803200 0 1 1]
+
 # BEGIN testcases8
 
 # Test parsing of ccyymmdd
-- 
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