test: add failing test case for recurring booking timezone bug

packages/lib/parse-dates.test.ts

@@ -0,0 +1,89 @@
+import { describe, expect, it } from "vitest";
+
+import dayjs from "@calcom/dayjs";
+
+import { parseRecurringDates } from "./parse-dates";
+
+describe("parseRecurringDates", () => {
+  describe("timezone handling for recurring events", () => {
+    it("should correctly generate recurring dates in CET timezone regardless of execution environment timezone", () => {
+      const userTimezone = "Europe/Paris"; // CET/CEST
+
+      const startDate = "2024-01-15T10:00:00"; // Time in user's local time (CET)
+
+      const recurringEvent = {
+        freq: 2, // WEEKLY (from rrule)
+        interval: 1,
+      };
+      const recurringCount = 3;
+
+      const [, dates] = parseRecurringDates(
+        {
+          startDate,
+          timeZone: userTimezone,
+          recurringEvent,
+          recurringCount,
+          withDefaultTimeFormat: true,
+        },
+        "en"
+      );
+
+      expect(dates).toHaveLength(3);
+
+      dates.forEach((date, index) => {
+        const dateInUserTz = dayjs(date).tz(userTimezone);
+        const hour = dateInUserTz.hour();
+        const minute = dateInUserTz.minute();
+
+        expect(hour).toBe(10);
+        expect(minute).toBe(0);
+
+        if (index > 0) {
+          const prevDate = dayjs(dates[index - 1]);
+          const daysDiff = dateInUserTz.diff(prevDate, "day");
+          expect(daysDiff).toBe(7);
+        }
+      });
+
+      expect(dayjs(dates[0]).tz(userTimezone).format("YYYY-MM-DD HH:mm")).toBe("2024-01-15 10:00");
+      expect(dayjs(dates[1]).tz(userTimezone).format("YYYY-MM-DD HH:mm")).toBe("2024-01-22 10:00");
+      expect(dayjs(dates[2]).tz(userTimezone).format("YYYY-MM-DD HH:mm")).toBe("2024-01-29 10:00");
+    });
+
+    it("should handle DST transitions correctly for recurring events in CET", () => {
+      const userTimezone = "Europe/Paris";
+
+      const startDate = "2024-03-18T10:00:00";
+
+      const recurringEvent = {
+        freq: 2, // WEEKLY
+        interval: 1,
+      };
+      const recurringCount = 3;
+
+      const [, dates] = parseRecurringDates(
+        {
+          startDate,
+          timeZone: userTimezone,
+          recurringEvent,
+          recurringCount,
+          withDefaultTimeFormat: true,
+        },
+        "en"
+      );
+
+      expect(dates).toHaveLength(3);
+
+      dates.forEach((date) => {
+        const dateInUserTz = dayjs(date).tz(userTimezone);
+        expect(dateInUserTz.hour()).toBe(10);
+        expect(dateInUserTz.minute()).toBe(0);
+      });
+
+      expect(dayjs(dates[0]).tz(userTimezone).format("YYYY-MM-DD HH:mm")).toBe("2024-03-18 10:00");
+      expect(dayjs(dates[1]).tz(userTimezone).format("YYYY-MM-DD HH:mm")).toBe("2024-03-25 10:00");
+      // Central European Time (CET) changes to Central European Summer Time (CEST) on 31 March 2024
+      expect(dayjs(dates[2]).tz(userTimezone).format("YYYY-MM-DD HH:mm")).toBe("2024-04-01 10:00");
+    });
+  });
+});

packages/lib/parse-dates.ts

@@ -99,14 +99,14 @@ export const parseRecurringDates = (
   const rule = new RRule({
     ...restRecurringEvent,
     count: recurringCount,
-    dtstart: new Date(dayjs(startDate).valueOf()),
+    dtstart: new Date(dayjs.tz(startDate, timeZone).valueOf()),
   });
 
-  const startUtcOffset = dayjs(startDate).utcOffset();
+  const startUtcOffset = dayjs.tz(startDate, timeZone).utcOffset();
   // UTC still need to have DST applied, rrule does not do this.
   const times = rule.all().map((t) => {
     // applying the DST offset.
-    return dayjs.utc(t).add(startUtcOffset - dayjs(t).utcOffset(), "minute");
+    return dayjs.utc(t).add(startUtcOffset - dayjs(t).tz(timeZone).utcOffset(), "minute");
   });
   const dateStrings = times.map((t) => {
     // finally; show in local timeZone again