fix bug in date->time-utc caused by incorrect use of difftime in Windows

original commit: 4748dad50c7f0e9b3ba002c27990d4c88c0189fc

LOG

@@ -772,3 +772,5 @@
     main.c
 - fix even? and odd? to error on exceptional flonums
     5_3.ss, 5_3.ms, fl.ms, root-experr*, patch*
+- fix bug in date->time-utc caused by incorrect use of difftime in Windows
+    stats.c, date.ms, release_notes.stex

c/stats.c

@@ -409,7 +409,7 @@ ptr S_mktime(ptr dtvec) {
 
   tzoff = adjust_time_zone(dtvec, &tmx, given_tzoff);
 
-  if (tzoff != orig_tzoff) tx = (time_t) difftime(tx, (time_t)(orig_tzoff - tzoff));
+  if (tzoff != orig_tzoff) tx = tx - orig_tzoff + tzoff;
 
   return Scons(S_integer_time_t(tx), Svector_ref(dtvec, dtvec_nsec));
 }

mats/date.ms

@@ -622,9 +622,11 @@
     (sleep (make-time 'time-duration 0 1))
     (time<? t (date->time-utc (current-date))))
   (let ([t (current-time)])
-    (time=?
-      (date->time-utc (time-utc->date t -14400))
-      (date->time-utc (time-utc->date t 0))))
+    (and
+     (time=? (date->time-utc (time-utc->date t)) t)
+     (time=? (date->time-utc (time-utc->date t -86400)) t)
+     (time=? (date->time-utc (time-utc->date t 0)) t)
+     (time=? (date->time-utc (time-utc->date t 86400)) t)))
 )
 
 (mat time&date-printing

release_notes/release_notes.stex

@@ -1535,6 +1535,14 @@ in fasl files does not generally make sense.
 %-----------------------------------------------------------------------------
 \section{Bug Fixes}\label{section:bugfixes}
 
+\subsection{Incorrect handling of offset in
+\protect\scheme{date->time-utc} on Windows (9.5)}
+
+A bug when \scheme{date->time-utc} is called on Windows with a
+date-zone-offset smaller than the system's time-zone offset has been
+fixed.
+[This bug dated back to Version 9.5.]
+
 \subsection{Compiler mishandling of fx /carry operations (9.5)}
 
 A bug in the source optimizer that caused an internal compiler error when