events: fix timezone-aware iCal export (local->UTC stamps) + 6 tests

Bug: tz events store wall-clock LOCAL times but export stamped them with a Z
(UTC) suffix, so a London 18:00 event falsely read as 18:00 UTC. ev-ical-conv
now converts a tz event's DTSTART/UNTIL/EXDATE/RDATE local->UTC before
formatting (London summer 18:00 -> 170000Z; Paris -> 160000Z); non-tz events
unchanged. Caveat: UTC RRULE drifts from wall-clock-stable tz recurrence across
a DST boundary (VTIMEZONE deferred). 366/366 green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/events/ical.sx

@@ -94,11 +94,27 @@
       (str (get e :ord) (ev-ical-wd (get e :wd)))
       (ev-ical-wd e))))
 
+;; A datetime converter for an event: tz-aware events store wall-clock LOCAL
+;; times, so export converts them to UTC (the `Z` stamps are absolute);
+;; non-tz events pass through unchanged.
+;;   CAVEAT: a UTC RRULE recurs at a fixed UTC offset, whereas a tz event's
+;;   expansion stays wall-clock-stable across DST — so for a tz recurrence that
+;;   crosses a DST boundary the exported series drifts by the offset change
+;;   after the boundary. DTSTART and each individual stamp are correct; full
+;;   fidelity would need a VTIMEZONE block (deferred).
+(define
+  ev-ical-conv
+  (fn
+    (event)
+    (let
+      ((tz (get event :tz)))
+      (if (nil? tz) (fn (t) t) (fn (t) (ev-tz-local->utc tz t))))))
+
 ;; ---- RRULE ----
 (define
   ev-ical-rrule
   (fn
-    (rrule)
+    (rrule conv)
     (let
       ((parts (list (str "FREQ=" (ev-ical-freq (get rrule :freq))))))
       (begin
@@ -112,7 +128,7 @@
           (append! parts (str "COUNT=" (get rrule :count))))
         (when
           (not (nil? (get rrule :until)))
-          (append! parts (str "UNTIL=" (ev-ical-dt (get rrule :until)))))
+          (append! parts (str "UNTIL=" (ev-ical-dt (conv (get rrule :until))))))
         (when
           (not (nil? (get rrule :byday)))
           (append!
@@ -139,17 +155,17 @@
   (fn
     (event)
     (let
-      ((lines (list "BEGIN:VEVENT")))
+      ((lines (list "BEGIN:VEVENT")) (conv (ev-ical-conv event)))
       (begin
         (append! lines (str "UID:" (get event :id)))
         (append! lines (str "SUMMARY:" (get event :id)))
-        (append! lines (str "DTSTART:" (ev-ical-dt (get event :dtstart))))
+        (append! lines (str "DTSTART:" (ev-ical-dt (conv (get event :dtstart)))))
         (append!
           lines
           (str "DURATION:" (ev-ical-duration (get event :duration))))
         (when
           (not (nil? (get event :rrule)))
-          (append! lines (ev-ical-rrule (get event :rrule))))
+          (append! lines (ev-ical-rrule (get event :rrule) conv)))
         (when
           (and
             (not (nil? (get event :exdate)))
@@ -158,7 +174,7 @@
             lines
             (str
               "EXDATE:"
-              (ev-ical-join (map ev-ical-dt (get event :exdate)) ","))))
+              (ev-ical-join (map (fn (d) (ev-ical-dt (conv d))) (get event :exdate)) ","))))
         (when
           (and
             (not (nil? (get event :rdate)))
@@ -167,7 +183,7 @@
             lines
             (str
               "RDATE:"
-              (ev-ical-join (map ev-ical-dt (get event :rdate)) ","))))
+              (ev-ical-join (map (fn (d) (ev-ical-dt (conv d))) (get event :rdate)) ","))))
         (append! lines "END:VEVENT")
         lines))))
 

lib/events/scoreboard.json

@@ -1,12 +1,12 @@
 {
   "lang": "events",
-  "total_passed": 360,
+  "total_passed": 366,
   "total_failed": 0,
-  "total": 360,
+  "total": 366,
   "suites": [
     {"name":"calendar","passed":51,"failed":0,"total":51},
     {"name":"timezone","passed":17,"failed":0,"total":17},
-    {"name":"ical","passed":40,"failed":0,"total":40},
+    {"name":"ical","passed":46,"failed":0,"total":46},
     {"name":"availability","passed":22,"failed":0,"total":22},
     {"name":"api","passed":41,"failed":0,"total":41},
     {"name":"booking","passed":82,"failed":0,"total":82},
@@ -17,5 +17,5 @@
     {"name":"federation","passed":29,"failed":0,"total":29},
     {"name":"integration","passed":8,"failed":0,"total":8}
   ],
-  "generated": "2026-06-07T17:28:07+00:00"
+  "generated": "2026-06-07T18:33:58+00:00"
 }

lib/events/scoreboard.md

@@ -1,12 +1,12 @@
 # events scoreboard
 
-**360 / 360 passing** (0 failure(s)).
+**366 / 366 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
 | calendar | 51 | 51 | ok |
 | timezone | 17 | 17 | ok |
-| ical | 40 | 40 | ok |
+| ical | 46 | 46 | ok |
 | availability | 22 | 22 | ok |
 | api | 41 | 41 | ok |
 | booking | 82 | 82 | ok |

lib/events/tests/ical.sx

@@ -266,6 +266,48 @@
               (ev-ic-starts (first events))
               (ev-ic-starts (ev-event (quote a) (ev-dt 2026 6 1 9 0) 30 {:freq :daily :count 3} 1)))))))))
 
+;; ---- timezone-aware export (local wall-clock -> UTC stamps) ----
+(define
+  ev-ic-dtstart
+  (fn (ev) (ev-ic-line (ev/event->ical-lines ev) "DTSTART")))
+
+(define
+  ev-ic-tz-run-all!
+  (fn
+    ()
+    (do
+      (ev-ic-check!
+        "London winter event exports as the same UTC (GMT)"
+        (ev-ic-dtstart (ev-event-tz (quote w) (ev-dt 2026 1 15 18 0) 60 nil 1 ev-tz-london))
+        "DTSTART:20260115T180000Z")
+      (ev-ic-check!
+        "London summer event exports one hour earlier in UTC (BST)"
+        (ev-ic-dtstart (ev-event-tz (quote s) (ev-dt 2026 7 15 18 0) 60 nil 1 ev-tz-london))
+        "DTSTART:20260715T170000Z")
+      (ev-ic-check!
+        "Paris winter (CET +1) exports one hour earlier in UTC"
+        (ev-ic-dtstart (ev-event-tz (quote p) (ev-dt 2026 1 15 18 0) 60 nil 1 ev-tz-paris))
+        "DTSTART:20260115T170000Z")
+      (ev-ic-check!
+        "Paris summer (CEST +2) exports two hours earlier in UTC"
+        (ev-ic-dtstart (ev-event-tz (quote p) (ev-dt 2026 7 15 18 0) 60 nil 1 ev-tz-paris))
+        "DTSTART:20260715T160000Z")
+      (ev-ic-check!
+        "a non-tz event is exported unchanged"
+        (ev-ic-dtstart (ev-event (quote n) (ev-dt 2026 7 15 18 0) 60 nil 1))
+        "DTSTART:20260715T180000Z")
+      ;; EXDATE on a tz event is also converted to UTC
+      (ev-ic-check!
+        "tz event EXDATE is converted to UTC"
+        (ev-ic-line
+          (ev/event->ical-lines
+            (assoc
+              (ev-event-tz (quote s) (ev-dt 2026 7 1 18 0) 60 {:freq :daily :count 3} 1 ev-tz-london)
+              :exdate
+              (list (ev-dt 2026 7 2 18 0))))
+          "EXDATE")
+        "EXDATE:20260702T170000Z"))))
+
 (define
   ev-ical-tests-run!
   (fn
@@ -276,4 +318,5 @@
       (set! ev-ic-failures (list))
       (ev-ic-run-all!)
       (ev-ic-rt-run-all!)
+      (ev-ic-tz-run-all!)
       {:failures ev-ic-failures :total (+ ev-ic-pass ev-ic-fail) :passed ev-ic-pass :failed ev-ic-fail})))

plans/events-on-sx.md

@@ -18,7 +18,7 @@ capacity rules, transactional booking, and a flow-driven notification dispatcher
 
 ## Status (rolling)
 
-`bash lib/events/conformance.sh` → **360/360** (Phases 1-4 + 13 ext: …iCalendar export+import round-trip, whole-series booking)
+`bash lib/events/conformance.sh` → **366/366** (Phases 1-4 + 13 ext + tz-aware iCal export fix)
 
 ## Ground rules
 
@@ -88,6 +88,13 @@ lib/events/api.sx ── (events/schedule) (events/book) (events/agenda) ──
 
 ## Progress log
 
+- 2026-06-07 — Fix: timezone-aware iCal export. Bug — tz events store wall-clock
+  LOCAL times, but export stamped them with a `Z` (UTC) suffix, so a London
+  18:00 event falsely read as 18:00 UTC. `ev-ical-conv` now converts a tz
+  event's DTSTART / UNTIL / EXDATE / RDATE local→UTC before formatting (London
+  summer 18:00 → 170000Z; Paris → 160000Z); non-tz events unchanged. Documented
+  caveat: a UTC RRULE drifts from a wall-clock-stable tz recurrence across a DST
+  boundary — full fidelity needs VTIMEZONE (deferred). +6 tests, 366/366 green.
 - 2026-06-07 — iCalendar import / round-trip (extension). `ical.sx` now parses
   VEVENT/VCALENDAR text back into events (`ev/ical-lines->event`,
   `ev/parse-vcalendar`): DTSTART/DURATION/RRULE (incl. ordinal BYDAY, BYMONTHDAY,