Fix wrong month in date picker calendar header

src/common/datetime/format_date.ts

@@ -294,3 +294,26 @@ export const formatCallyDateRange = (
 
   return `${startDate}/${endDate}`;
 };
+
+/**
+ * August 2021, for calendar days coming out of cally.
+ *
+ * cally hands out calendar days as `Date` objects anchored to UTC midnight, and
+ * formats its own headings in UTC too. Anything derived from a cally event has
+ * to be formatted the same way: the resolved time zone shifts the day back for
+ * negative UTC offsets, which becomes a wrong month — and in January a wrong
+ * year — whenever the day is the 1st.
+ */
+export const formatCallyMonthYear = (
+  dateObj: Date,
+  locale: FrontendLocaleData
+) => formatCallyMonthYearMem(locale.language).format(dateObj);
+
+const formatCallyMonthYearMem = memoizeOne(
+  (language: string) =>
+    new Intl.DateTimeFormat(language, {
+      month: "long",
+      year: "numeric",
+      timeZone: "UTC",
+    })
+);

src/components/date-picker/date-range-picker.ts

@@ -9,8 +9,8 @@ import { customElement, property, queryAll, state } from "lit/decorators";
 import { firstWeekdayIndex } from "../../common/datetime/first_weekday";
 import {
   formatCallyDateRange,
-  formatDateMonth,
-  formatDateYear,
+  formatCallyMonthYear,
+  formatDateMonthYear,
   formatISODateOnly,
 } from "../../common/datetime/format_date";
 import { transform } from "../../common/decorators/transform";
@@ -56,13 +56,16 @@ export class DateRangePicker extends MobileAwareMixin(LitElement) {
   })
   private _hassConfig!: HassConfig;
 
-  /** used to show month in calendar-range header */
-  @state() private _pickerMonth?: string;
+  /** used to show month and year in calendar-range header */
+  @state() private _pickerMonthYear?: string;
 
-  /** used to show year in calendar-date header */
-  @state() private _pickerYear?: string;
-
-  /** used for today to navigate focus in calendar-range  */
+  /**
+   * used for today to navigate focus in calendar-range
+   *
+   * Always mirrors the day calendar-range has focused, never cleared: once this
+   * has held a date, rendering `undefined` over it makes cally fall back to the
+   * selected range and page the calendar away from where the user is.
+   */
   @state() private _focusDate?: string;
 
   @state() private _dateValue?: string;
@@ -88,12 +91,7 @@ export class DateRangePicker extends MobileAwareMixin(LitElement) {
             this._hassConfig
           )
         : undefined;
-    this._pickerMonth = formatDateMonth(
-      date,
-      this._i18n.locale,
-      this._hassConfig
-    );
-    this._pickerYear = formatDateYear(
+    this._pickerMonthYear = formatDateMonthYear(
       date,
       this._i18n.locale,
       this._hassConfig
@@ -163,9 +161,7 @@ export class DateRangePicker extends MobileAwareMixin(LitElement) {
               slot="previous"
             ></ha-icon-button-prev>
             <div class="heading" slot="heading">
-              <span class="month-year"
-                >${this._pickerMonth} ${this._pickerYear}</span
-              >
+              <span class="month-year">${this._pickerMonthYear}</span>
               <ha-icon-button
                 @click=${this._focusToday}
                 .path=${mdiCalendarToday}
@@ -229,12 +225,7 @@ export class DateRangePicker extends MobileAwareMixin(LitElement) {
       this._i18n.locale,
       this._hassConfig
     );
-    this._pickerMonth = formatDateMonth(
-      date,
-      this._i18n.locale,
-      this._hassConfig
-    );
-    this._pickerYear = formatDateYear(
+    this._pickerMonthYear = formatDateMonthYear(
       date,
       this._i18n.locale,
       this._hassConfig
@@ -308,24 +299,15 @@ export class DateRangePicker extends MobileAwareMixin(LitElement) {
   }
 
   private _focusChanged(ev: CustomEvent<Date>) {
-    const date = ev.detail;
-    this._pickerMonth = formatDateMonth(
-      date,
-      this._i18n.locale,
-      this._hassConfig
-    );
-    this._pickerYear = formatDateYear(
-      date,
-      this._i18n.locale,
-      this._hassConfig
-    );
-    this._focusDate = undefined;
+    const dateElement = ev.target as HTMLElementTagNameMap["calendar-range"];
+    this._pickerMonthYear = formatCallyMonthYear(ev.detail, this._i18n.locale);
+    this._focusDate = dateElement.focusedDate;
   }
 
   private _handleChange(ev: CustomEvent) {
     const dateElement = ev.target as HTMLElementTagNameMap["calendar-range"];
     this._dateValue = dateElement.value;
-    this._focusDate = undefined;
+    this._focusDate = dateElement.focusedDate;
   }
 
   private _clickDateRangeChip(ev: Event) {

src/components/date-picker/ha-dialog-date-picker.ts

@@ -6,7 +6,8 @@ import type { HassConfig } from "home-assistant-js-websocket/dist/types";
 import { css, html, LitElement, nothing } from "lit";
 import { customElement, state } from "lit/decorators";
 import {
-  formatDateMonth,
+  formatCallyMonthYear,
+  formatDateMonthYear,
   formatDateShort,
   formatDateYear,
   formatISODateOnly,
@@ -56,13 +57,16 @@ export class HaDialogDatePicker extends DialogMixin<DatePickerDialogParams>(
     dateString: string;
   };
 
-  /** used to show month in calendar-date header */
-  @state() private _pickerMonth?: string;
+  /** used to show month and year in calendar-date header */
+  @state() private _pickerMonthYear?: string;
 
-  /** used to show year in calendar-date header */
-  @state() private _pickerYear?: string;
-
-  /** used for today to navigate focus in cally-calendar-date  */
+  /**
+   * used for today to navigate focus in cally-calendar-date
+   *
+   * Always mirrors the day calendar-date has focused, never cleared: once this
+   * has held a date, rendering `undefined` over it makes cally fall back to the
+   * selected value and page the calendar away from where the user is.
+   */
   @state() private _focusDate?: string;
 
   public connectedCallback() {
@@ -71,12 +75,7 @@ export class HaDialogDatePicker extends DialogMixin<DatePickerDialogParams>(
     if (this.params) {
       const date = valueToDate(this.params.value);
 
-      this._pickerYear = formatDateYear(
-        date,
-        this._i18n.locale,
-        this._hassConfig
-      );
-      this._pickerMonth = formatDateMonth(
+      this._pickerMonthYear = formatDateMonthYear(
         date,
         this._i18n.locale,
         this._hassConfig
@@ -84,7 +83,7 @@ export class HaDialogDatePicker extends DialogMixin<DatePickerDialogParams>(
 
       this._value = this.params.value
         ? {
-            year: this._pickerYear,
+            year: formatDateYear(date, this._i18n.locale, this._hassConfig),
             title: formatDateShort(date, this._i18n.locale, this._hassConfig),
             dateString: formatISODateOnly(
               date,
@@ -140,9 +139,7 @@ export class HaDialogDatePicker extends DialogMixin<DatePickerDialogParams>(
           slot="previous"
         ></ha-icon-button-prev>
         <div class="heading" slot="heading">
-          <span class="month-year"
-            >${this._pickerMonth} ${this._pickerYear}</span
-          >
+          <span class="month-year">${this._pickerMonthYear}</span>
           <ha-icon-button
             @click=${this._setToday}
             .path=${mdiCalendarToday}
@@ -185,12 +182,7 @@ export class HaDialogDatePicker extends DialogMixin<DatePickerDialogParams>(
 
     if (setFocusDay) {
       this._focusDate = this._value.dateString;
-      this._pickerMonth = formatDateMonth(
-        date,
-        this._i18n.locale,
-        this._hassConfig
-      );
-      this._pickerYear = formatDateYear(
+      this._pickerMonthYear = formatDateMonthYear(
         date,
         this._i18n.locale,
         this._hassConfig
@@ -199,18 +191,9 @@ export class HaDialogDatePicker extends DialogMixin<DatePickerDialogParams>(
   }
 
   private _focusChanged(ev: CustomEvent<Date>) {
-    const date = ev.detail;
-    this._pickerMonth = formatDateMonth(
-      date,
-      this._i18n.locale,
-      this._hassConfig
-    );
-    this._pickerYear = formatDateYear(
-      date,
-      this._i18n.locale,
-      this._hassConfig
-    );
-    this._focusDate = undefined;
+    const dateElement = ev.target as CalendarDate;
+    this._pickerMonthYear = formatCallyMonthYear(ev.detail, this._i18n.locale);
+    this._focusDate = dateElement.focusedDate;
   }
 
   private _clear() {

test/common/datetime/format_date.test.ts

@@ -1,5 +1,6 @@
 import { describe, expect, it } from "vitest";
 import {
+  formatCallyMonthYear,
   formatDate,
   formatDateWeekdayDay,
   formatDateShort,
@@ -263,4 +264,43 @@ describe("formatDate", () => {
       ).toBe("Sat");
     });
   });
+
+  describe("formatCallyMonthYear", () => {
+    // How cally hands out a calendar day: a Date anchored to UTC midnight
+    const julyFirst = new Date(Date.UTC(2026, 6, 1));
+    const januaryFirst = new Date(Date.UTC(2026, 0, 1));
+
+    // demoConfig.time_zone is America/Los_Angeles, so this resolves to a
+    // negative UTC offset whatever zone the tests run in
+    const locale = {
+      language: "en",
+      number_format: NumberFormat.language,
+      time_format: TimeFormat.language,
+      date_format: DateFormat.language,
+      time_zone: TimeZone.server,
+      first_weekday: FirstWeekday.language,
+    };
+
+    it("Formats in UTC rather than the resolved time zone", () => {
+      // A time zone aware formatter reads UTC midnight on the 1st as the
+      // previous month, which desyncs the header from the calendar grid
+      expect(formatDateMonthYear(julyFirst, locale, demoConfig)).toBe(
+        "June 2026"
+      );
+      expect(formatCallyMonthYear(julyFirst, locale)).toBe("July 2026");
+    });
+
+    it("Keeps the year on a January page", () => {
+      expect(formatDateMonthYear(januaryFirst, locale, demoConfig)).toBe(
+        "December 2025"
+      );
+      expect(formatCallyMonthYear(januaryFirst, locale)).toBe("January 2026");
+    });
+
+    it("Orders month and year by locale", () => {
+      expect(
+        formatCallyMonthYear(julyFirst, { ...locale, language: "ja" })
+      ).toBe("2026年7月");
+    });
+  });
 });