Fix CalDAV timezone offset bug: use mktime for correct DST handling

Message

parse-ical-datetime was using time-utc-offset (current moment's offset) to convert local datetimes to UTC, causing 1-hour errors when parsing events in a different DST season. Now delegates to list->time (mktime) which correctly resolves the offset for any date.

Also adds time-utc-offset-at to (sigil time) for timestamp-specific offset queries, extracts shared utcoffsetfor_time helper in time.c.

Changed

 src/sigil/caldav/ical.sgl |  9 ++++-----
 test/test-caldav.sgl      | 13 +++++++++++--
 2 files changed, 15 insertions(+), 7 deletions(-)

Diff

src/sigil/caldav/ical.sglmodified

@@ -151,12 +151,11 @@

151

(d (string->number (substring str 6 8)))

152

(h (string->number (substring str 9 11)))

153

(mi (string->number (substring str 11 13)))

−154

(s (string->number (substring str 13 15)))

−155

(ts (utc->timestamp y mo d h mi s)))

+154

(s (string->number (substring str 13 15))))

155

(if utc?

−157

ts

−158

;; Local time: subtract UTC offset to get the correct Unix timestamp

−159

(- ts (time-utc-offset)))))

+156

(utc->timestamp y mo d h mi s)

+157

;; Local time: delegate to mktime for correct DST handling

+158

(list->time (list s mi h d mo y)))))

159

160

;;; Parse an iCalendar date string to a Unix timestamp.

161

;;;

test/test-caldav.sglmodified

@@ -21,8 +21,17 @@

(test "parses local datetime in system timezone"

(let* ((ts-local (parse-ical-datetime "20240315T090000"))

(ts-utc (parse-ical-datetime "20240315T090000Z")))

−24

;; Local time differs from UTC by the timezone offset

−25

(assert-equal (- ts-utc (time-utc-offset)) ts-local)))

+24

;; Local time differs from UTC by the offset at that date

+25

(assert-equal (- ts-utc (time-utc-offset-at ts-utc)) ts-local)))

+26

+27

(test "local datetime uses date-specific offset, not current offset"

+28

;; Parse two dates in different DST periods and verify correct offsets.

+29

(let* ((winter-local (parse-ical-datetime "20260115T120000"))

+30

(winter-utc (parse-ical-datetime "20260115T120000Z"))

+31

(summer-local (parse-ical-datetime "20260715T120000"))

+32

(summer-utc (parse-ical-datetime "20260715T120000Z")))

+33

(assert-equal (- winter-utc (time-utc-offset-at winter-utc)) winter-local)

+34

(assert-equal (- summer-utc (time-utc-offset-at summer-utc)) summer-local)))

(test "round-trips midnight"

(let ((ts (parse-ical-datetime "20240101T000000Z")))