fix(diff-schedule): resolve DST offset bug

supabase/functions/diff-schedule/helpers.test.ts

@@ -37,3 +37,19 @@ Deno.test("localToUtc converts midnight correctly", () => {
   const result = localToUtc("2026-07-11", "00:00", "Europe/Lisbon");
   assertEquals(result, "2026-07-10T23:00:00.000Z");
 });
+
+Deno.test("localToUtc resolves a spring-forward wall time inside the skipped hour", () => {
+  // Lisbon clocks jump from 01:00 to 02:00 local at 2026-03-29T01:00:00Z, so
+  // "01:30" never occurs. @date-fns/tz's TZDate resolves a wall time inside
+  // the skipped hour using the pre-transition (+00:00) offset, i.e. as if
+  // DST had not yet started.
+  const result = localToUtc("2026-03-29", "01:30", "Europe/Lisbon");
+  assertEquals(result, "2026-03-29T01:30:00.000Z");
+});
+
+Deno.test("localToUtc resolves a fall-back wall time inside the repeated hour", () => {
+  // Lisbon clocks fall back from 02:00 to 01:00 local at 2026-10-25T01:00:00Z,
+  // so "01:30" occurs twice. Resolve to the later (post-transition) instant.
+  const result = localToUtc("2026-10-25", "01:30", "Europe/Lisbon");
+  assertEquals(result, "2026-10-25T01:30:00.000Z");
+});

supabase/functions/diff-schedule/helpers.ts

@@ -1,3 +1,5 @@
+import { TZDate } from "npm:@date-fns/tz@1.5.0";
+
 export function toSlug(name: string): string {
   return name
     .toLowerCase()
@@ -21,14 +23,10 @@ export function localToUtc(
   timeStr: string,
   timezone: string,
 ): string {
-  const localIso = `${dateStr}T${timeStr}:00`;
-  const naiveUtc = new Date(localIso + "Z");
-  // sv-SE locale gives "YYYY-MM-DD HH:MM:SS" — unambiguously parseable as UTC
-  const localInTz = new Date(
-    naiveUtc.toLocaleString("sv-SE", { timeZone: timezone }) + "Z",
-  );
-  const offsetMs = naiveUtc.getTime() - localInTz.getTime();
-  return new Date(naiveUtc.getTime() + offsetMs).toISOString();
+  const [year, month, day] = dateStr.split("-").map(Number);
+  const [hour, minute] = timeStr.split(":").map(Number);
+  const zoned = new TZDate(year, month - 1, day, hour, minute, 0, timezone);
+  return new Date(+zoned).toISOString();
 }
 
 export function utcToLocalDate(utcIso: string, timezone: string): string {