CTest: adopt std::chrono::system_clock

After the refactor to make CTest use std::chrono::steady_clock for the
keeping of time for test duration, there are still references to
cmSystemTools::GetTime() left.
To further adopt std::chrono for time related activities, this commit
changes those remaining references to std::chrono::system_clock::now()
calls and alters the storage from either unsigned int or double to
std::chrono::system_clock::time_point.

For ease of conversion, a converter method is added to cmXMLWriter that
converts from a std::chrono::system_clock::time_point to the number of
seconds since the UN*X epoch as that is expected behaviour. This means
no more casts as required.

Functionally should be no difference as the system_clock is implemented
in the same terms.

1 files changed, 18 insertions, 0 deletions

diff --git a/Source/cmXMLWriter.h b/Source/cmXMLWriter.h
index 981255d..c890acf 100644
--- a/Source/cmXMLWriter.h
+++ b/Source/cmXMLWriter.h
@@ -7,6 +7,8 @@
 
 #include "cmXMLSafe.h"
 
+#include <chrono>
+#include <ctime>
 #include <ostream>
 #include <stack>
 #include <string>
@@ -99,6 +101,22 @@ private:
     return cmXMLSafe(value).Quotes(false);
   }
 
+  /*
+   * Convert a std::chrono::system::time_point to the number of seconds since
+   * the UN*X epoch.
+   *
+   * It would be tempting to convert a time_point to number of seconds by
+   * using time_since_epoch(). Unfortunately the C++11 standard does not
+   * specify what the epoch of the system_clock must be.
+   * Therefore we must assume it is an arbitary point in time. Instead of this
+   * method, it is recommended to convert it by means of the to_time_t method.
+   */
+  static std::time_t SafeContent(
+    std::chrono::system_clock::time_point const& value)
+  {
+    return std::chrono::system_clock::to_time_t(value);
+  }
+
   template <typename T>
   static T SafeContent(T value)
   {