Fix DST day generation for specific-times events

frontend/src/components/schedule_overlap/ScheduleOverlap.vue

@@ -1019,6 +1019,7 @@ import {
   _delete,
   get,
   getDateDayOffset,
+  getSpecificTimesDayStarts,
   isDateBetween,
   generateEnabledCalendarsPayload,
   isTouchEnabled,
@@ -1477,35 +1478,17 @@ export default {
         (this.state === this.states.SET_SPECIFIC_TIMES ||
           this.event.times?.length === 0)
       ) {
-        let prevDate = null // Stores the prevDate to check if the current date is consecutive to the previous date
-        for (let i = 0; i < this.event.dates.length; ++i) {
-          const date = new Date(this.event.dates[i])
-          const localDate = new Date(
-            date.getTime() - this.timezoneOffset * 60 * 1000
-          )
-          localDate.setUTCHours(0, 0, 0, 0)
-          localDate.setTime(
-            localDate.getTime() + this.timezoneOffset * 60 * 1000
-          )
-
-          if (!datesSoFar.has(localDate.getTime())) {
-            datesSoFar.add(localDate.getTime())
-
-            let isConsecutive = true
-            if (prevDate) {
-              isConsecutive =
-                prevDate.getTime() === localDate.getTime() - 24 * 60 * 60 * 1000
-            }
-            const { dayString, dateString } = getDateString(localDate)
-            days.push({
-              dayText: dayString,
-              dateString,
-              dateObject: localDate,
-              isConsecutive,
-            })
-
-            prevDate = new Date(localDate)
-          }
+        for (const day of getSpecificTimesDayStarts(
+          this.event.dates,
+          this.curTimezone
+        )) {
+          const { dayString, dateString } = getDateString(day.dateObject)
+          days.push({
+            dayText: dayString,
+            dateString,
+            dateObject: day.dateObject,
+            isConsecutive: day.isConsecutive,
+          })
         }
         return days
       }

frontend/src/utils/date_utils.js

@@ -227,6 +227,51 @@ export const getScheduleTimezoneOffset = (
     getTimezoneReferenceDateForEvent(event, weekOffset)
   )
 }
+const getDateInTimezone = (date, curTimezone) => {
+  if (curTimezone?.value) {
+    return dayjs(date).tz(curTimezone.value)
+  }
+
+  if ("offset" in curTimezone) {
+    return dayjs(date).utcOffset(curTimezone.offset)
+  }
+
+  return dayjs(date)
+}
+
+/** Returns the unique day-start datetimes for specific-times events */
+export const getSpecificTimesDayStarts = (eventDates, curTimezone) => {
+  const days = []
+  const datesSoFar = new Set()
+  let prevDay = null
+
+  for (const eventDate of eventDates) {
+    const localDate = getDateInTimezone(eventDate, curTimezone)
+      .startOf("day")
+      .toDate()
+
+    if (!datesSoFar.has(localDate.getTime())) {
+      datesSoFar.add(localDate.getTime())
+
+      let isConsecutive = true
+      if (prevDay) {
+        isConsecutive = prevDay.add(1, "day").isSame(
+          getDateInTimezone(localDate, curTimezone),
+          "day"
+        )
+      }
+
+      days.push({
+        dateObject: localDate,
+        isConsecutive,
+      })
+
+      prevDay = getDateInTimezone(localDate, curTimezone)
+    }
+  }
+
+  return days
+}
 
 /**
  * Returns a date, transformed to be in the same week of the dows array.

frontend/src/utils/date_utils.test.js

@@ -1,10 +1,17 @@
 import { beforeEach, describe, expect, it, vi } from "vitest"
 import {
   getScheduleTimezoneOffset,
+  getSpecificTimesDayStarts,
   getTimezoneOffsetForDate,
   getTimezoneReferenceDateForEvent,
 } from "./date_utils"
 import { eventTypes } from "../constants"
+import dayjs from "dayjs"
+import utcPlugin from "dayjs/plugin/utc"
+import timezonePlugin from "dayjs/plugin/timezone"
+
+dayjs.extend(utcPlugin)
+dayjs.extend(timezonePlugin)
 
 describe("DST timezone regression", () => {
   beforeEach(() => {
@@ -113,3 +120,90 @@ describe("DST timezone regression", () => {
     ).toBe(-60)
   })
 })
+
+describe("specific-times DST regression", () => {
+  it("falls back to the browser timezone when curTimezone is empty", () => {
+    const eventDates = [
+      "2026-01-12",
+      "2026-01-13",
+      "2026-01-14",
+      "2026-01-15",
+    ].map((day) => dayjs.tz(`${day} 00:00`, "America/Los_Angeles").toDate())
+
+    const expectedDates = [
+      ...new Set(
+        eventDates.map((eventDate) =>
+          dayjs(eventDate).startOf("day").toDate().toISOString()
+        )
+      ),
+    ]
+
+    expect(
+      getSpecificTimesDayStarts(eventDates, {}).map((day) =>
+        day.dateObject.toISOString()
+      )
+    ).toEqual(expectedDates)
+  })
+  it("keeps one civil day per selected date during non-DST periods", () => {
+    const timezone = "America/Los_Angeles"
+    const eventDates = [
+      "2026-01-12",
+      "2026-01-13",
+      "2026-01-14",
+      "2026-01-15",
+    ].map((day) => dayjs.tz(`${day} 00:00`, timezone).toDate())
+
+    expect(
+      getSpecificTimesDayStarts(eventDates, { value: timezone }).map((day) =>
+        day.dateObject.toISOString()
+      )
+    ).toEqual([
+      "2026-01-12T08:00:00.000Z",
+      "2026-01-13T08:00:00.000Z",
+      "2026-01-14T08:00:00.000Z",
+      "2026-01-15T08:00:00.000Z",
+    ])
+  })
+
+  it("keeps one civil day per selected date across spring-forward DST changes", () => {
+    const timezone = "America/Los_Angeles"
+    const eventDates = [
+      "2026-03-07",
+      "2026-03-08",
+      "2026-03-09",
+      "2026-03-10",
+    ].map((day) => dayjs.tz(`${day} 00:00`, timezone).toDate())
+
+    expect(
+      getSpecificTimesDayStarts(eventDates, { value: timezone }).map((day) =>
+        day.dateObject.toISOString()
+      )
+    ).toEqual([
+      "2026-03-07T08:00:00.000Z",
+      "2026-03-08T08:00:00.000Z",
+      "2026-03-09T07:00:00.000Z",
+      "2026-03-10T07:00:00.000Z",
+    ])
+  })
+
+  it("keeps one civil day per selected date across fall-back DST changes", () => {
+    const timezone = "America/Los_Angeles"
+    const eventDates = [
+      "2026-10-31",
+      "2026-11-01",
+      "2026-11-02",
+      "2026-11-03",
+    ].map((day) => dayjs.tz(`${day} 00:00`, timezone).toDate())
+
+    expect(
+      getSpecificTimesDayStarts(eventDates, { value: timezone }).map((day) =>
+        day.dateObject.toISOString()
+      )
+    ).toEqual([
+      "2026-10-31T07:00:00.000Z",
+      "2026-11-01T07:00:00.000Z",
+      "2026-11-02T08:00:00.000Z",
+      "2026-11-03T08:00:00.000Z",
+    ])
+  })
+})