date-range-picker with cally (#30193)

* date-range-picker with cally

* fix timePicker

* Review: backdrop transition

* fix comments

* Add formatCallyDateRange

* Refactor date formatting in date range picker and remove unused styles

* time-input without label

* review

src/common/datetime/format_date.ts

@@ -261,3 +261,36 @@ const formatDateWeekdayShortDateMem = memoizeOne(
       timeZone: resolveTimeZone(locale.time_zone, serverTimeZone),
     })
 );
+
+/**
+ * Format a date as YYYY-MM-DD. Uses "en-CA" because it's the only
+ * Intl locale that natively outputs ISO 8601 date format.
+ * Locale/config are only used to resolve the time zone.
+ */
+export const formatISODateOnly = (
+  dateObj: Date,
+  locale: FrontendLocaleData,
+  config: HassConfig
+) => {
+  const timeZone = resolveTimeZone(locale.time_zone, config.time_zone);
+  const formatter = new Intl.DateTimeFormat("en-CA", {
+    year: "numeric",
+    month: "2-digit",
+    day: "2-digit",
+    timeZone,
+  });
+  return formatter.format(dateObj);
+};
+
+// 2026-08-10/2026-08-15
+export const formatCallyDateRange = (
+  start: Date,
+  end: Date,
+  locale: FrontendLocaleData,
+  config: HassConfig
+) => {
+  const startDate = formatISODateOnly(start, locale, config);
+  const endDate = formatISODateOnly(end, locale, config);
+
+  return `${startDate}/${endDate}`;
+};