https://github.com/python/cpython/commit/170d85a37deb77a1711ab86288a33fda5c0a381a
commit: 170d85a37deb77a1711ab86288a33fda5c0a381a
branch: main
author: Hugo van Kemenade <[email protected]>
committer: encukou <[email protected]>
date: 2026-03-10T09:45:07+01:00
summary:
gh-145731: Fix negative timestamp during DST on Windows (GH-145728)
files:
A Misc/NEWS.d/next/Windows/2026-03-10-09-46-44.gh-issue-145731.5uEGgb.rst
M Python/pytime.c
diff --git
a/Misc/NEWS.d/next/Windows/2026-03-10-09-46-44.gh-issue-145731.5uEGgb.rst
b/Misc/NEWS.d/next/Windows/2026-03-10-09-46-44.gh-issue-145731.5uEGgb.rst
new file mode 100644
index 00000000000000..676a68e5a912f5
--- /dev/null
+++ b/Misc/NEWS.d/next/Windows/2026-03-10-09-46-44.gh-issue-145731.5uEGgb.rst
@@ -0,0 +1 @@
+Fix negative timestamp during DST on Windows. Patch by Hugo van Kemenade.
diff --git a/Python/pytime.c b/Python/pytime.c
index 2b1488911ef97b..399ff59ad01ab6 100644
--- a/Python/pytime.c
+++ b/Python/pytime.c
@@ -320,23 +320,27 @@ _PyTime_windows_filetime(time_t timer, struct tm *tm, int
is_local)
ft.dwLowDateTime = (DWORD)(ticks); // cast to DWORD truncates to low 32
bits
ft.dwHighDateTime = (DWORD)(ticks >> 32);
- /* Convert FILETIME to SYSTEMTIME */
+ /* Convert FILETIME to SYSTEMTIME (UTC) */
+ SYSTEMTIME st_utc;
+ if (!FileTimeToSystemTime(&ft, &st_utc)) {
+ PyErr_SetFromWindowsErr(0);
+ return -1;
+ }
+
SYSTEMTIME st_result;
if (is_local) {
- /* Convert to local time */
- FILETIME ft_local;
- if (!FileTimeToLocalFileTime(&ft, &ft_local) ||
- !FileTimeToSystemTime(&ft_local, &st_result)) {
+ /* Convert UTC SYSTEMTIME to local SYSTEMTIME.
+ * We use SystemTimeToTzSpecificLocalTime instead of
+ * FileTimeToLocalFileTime because the latter always applies the
+ * _current_ DST bias, whereas the former applies the correct
+ * DST rules for the date being converted (gh-80620). */
+ if (!SystemTimeToTzSpecificLocalTime(NULL, &st_utc, &st_result)) {
PyErr_SetFromWindowsErr(0);
return -1;
}
}
else {
- /* Convert to UTC */
- if (!FileTimeToSystemTime(&ft, &st_result)) {
- PyErr_SetFromWindowsErr(0);
- return -1;
- }
+ st_result = st_utc;
}
/* Convert SYSTEMTIME to struct tm */
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