Fix cross-timezone event parsing using TZID parameter

Message

parse-ical-datetime now accepts an optional timezone argument. When a TZID is present on DTSTART/DTEND, parse-dt-value extracts it and passes it through, so events in non-system timezones are parsed correctly.

Adds tests for cross-timezone parsing including DST transitions.

Changed

 src/sigil/caldav/ical.sgl | 29 ++++++++++++++++++++---------
 test/test-caldav.sgl      | 35 ++++++++++++++++++++++++++++++++++-
 2 files changed, 54 insertions(+), 10 deletions(-)

Diff

src/sigil/caldav/ical.sglmodified

@@ -137,25 +137,33 @@

137

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

138

;;;

139

;;; UTC datetimes (Z suffix) are interpreted as UTC.

−140

;;; Local datetimes (no Z) are interpreted in the system timezone.

+140

;;; Local datetimes (no Z) are interpreted in the given timezone,

+141

;;; or the system timezone if no timezone is provided.

142

;;;

143

;;; ```scheme

144

;;; (parse-ical-datetime "20240315T090000Z") ; => UTC 09:00

145

;;; (parse-ical-datetime "20240315T090000") ; => local 09:00

+146

;;; (parse-ical-datetime "20240315T090000" "America/New_York") ; => Eastern 09:00

147

;;; ```

−146

(define (parse-ical-datetime str)

+148

(define (parse-ical-datetime str . rest)

149

(: string? -> number?)

−148

(let* ((utc? (string-ends-with? str "Z"))

+150

(let* ((tz (if (null? rest) #f (car rest)))

+151

(utc? (string-ends-with? str "Z"))

152

(y (string->number (substring str 0 4)))

153

(mo (string->number (substring str 4 6)))

154

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

155

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

156

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

157

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

−155

(if utc?

−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)))))

+158

(cond

+159

(utc?

+160

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

+161

(tz

+162

;; Use the event's timezone for correct cross-timezone parsing

+163

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

+164

(else

+165

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

+166

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

167

168

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

169

;;;

@@ -294,11 +302,14 @@

302

((and (char=? c #\:) (not in-quotes?)) i)

303

(else (loop (+ i 1) in-quotes?))))))))

304

−297

;; Parse a DTSTART or DTEND property, checking params for VALUE=DATE

+305

;; Parse a DTSTART or DTEND property, checking params for VALUE=DATE.

+306

;; RFC 5545 requires local-form datetimes to be interpreted in the

+307

;; accompanying TZID, not the system timezone.

308

(define (parse-dt-value params value)

309

(if (string-contains? (string-upcase params) "VALUE=DATE")

310

(cons (parse-ical-date value) #t)

−301

(cons (parse-ical-datetime value) #f)))

+311

(let ((tzid (extract-tzid params)))

+312

(cons (parse-ical-datetime value tzid) #f))))

313

314

;; Extract TZID value from parameter string, e.g. "TZID=Europe/Athens"

315

(define (extract-tzid params)

test/test-caldav.sglmodified

@@ -62,7 +62,40 @@

(assert-equal 1710493200 (parse-ical-datetime "20240315T090000Z")))

(test "epoch is correct"

−65

(assert-equal 0 (parse-ical-datetime "19700101T000000Z"))))

+65

(assert-equal 0 (parse-ical-datetime "19700101T000000Z")))

+66

+67

(test "parses datetime with explicit timezone"

+68

;; 11:00 America/New_York on 2026-03-26 is EDT (UTC-4) = 15:00 UTC

+69

(let ((ts (parse-ical-datetime "20260326T110000" "America/New_York")))

+70

(assert-equal "20260326T150000Z" (format-ical-datetime ts))))

+71

+72

(test "parses datetime with different timezone than system"

+73

;; 09:00 America/Los_Angeles on 2026-03-26 is PDT (UTC-7) = 16:00 UTC

+74

(let ((ts (parse-ical-datetime "20260326T090000" "America/Los_Angeles")))

+75

(assert-equal "20260326T160000Z" (format-ical-datetime ts))))

+76

+77

(test "timezone param ignored for UTC datetimes"

+78

;; Z suffix means UTC regardless of any timezone hint

+79

(let ((ts (parse-ical-datetime "20260326T150000Z" "America/New_York")))

+80

(assert-equal "20260326T150000Z" (format-ical-datetime ts))))

+81

+82

(test "timezone handles DST correctly"

+83

;; America/New_York: EST (UTC-5) in January, EDT (UTC-4) in July

+84

(let ((winter (parse-ical-datetime "20260115T120000" "America/New_York"))

+85

(summer (parse-ical-datetime "20260715T120000" "America/New_York")))

+86

;; Winter: 12:00 EST = 17:00 UTC

+87

(assert-equal "20260115T170000Z" (format-ical-datetime winter))

+88

;; Summer: 12:00 EDT = 16:00 UTC

+89

(assert-equal "20260715T160000Z" (format-ical-datetime summer))))

+90

+91

(test "cross-timezone event parsed correctly via ical-parse"

+92

    (let* ((ical-text "BEGIN:VCALENDAR\r\nVERSION:2.0\r\nBEGIN:VEVENT\r\nUID:[email protected]\r\nDTSTART;TZID=America/New_York:20260326T110000\r\nDTEND;TZID=America/New_York:20260326T120000\r\nSUMMARY:Cross-TZ Meeting\r\nEND:VEVENT\r\nEND:VCALENDAR")

+93

(events (ical-parse ical-text))

+94

(ev (car events)))

+95

;; 11:00 EDT = 15:00 UTC, 12:00 EDT = 16:00 UTC

+96

(assert-equal "20260326T150000Z" (format-ical-datetime (ical-event-dtstart ev)))

+97

(assert-equal "20260326T160000Z" (format-ical-datetime (ical-event-dtend ev)))

+98

(assert-equal "America/New_York" (ical-event-timezone ev)))))

100

101

;; ============================================================