feat: extend useFormattedISODate helper function for advanced use cases

* can now be used to represent date, time and datetime strings
* does support nullish-values with proper types
  (and adds a corrosponding default value)
* allows timezone offset to be stripped for use
  in datetime-local input fields

src/util.ts

@@ -60,13 +60,54 @@ export function useAsyncFunction<F extends (...args: never[]) => Promise<unknown
   return { fn: wrapper, error, isProcessing }
 }
 
-export function useFormattedISODate(date: Ref<Date>) {
+type UseFormattedISODateOptions = {
+  representation?: 'date' | 'time' | 'complete' | undefined
+  stripOffset?: boolean | undefined
+}
+
+export function useFormattedISODate(
+  date: Ref<Date>,
+  defaultValue?: MaybeRefOrGetter<Date>,
+  options?: UseFormattedISODateOptions | undefined,
+): ComputedRef<string>
+export function useFormattedISODate(
+  date: Ref<Date | null | undefined>,
+  defaultValue?: MaybeRefOrGetter<Date>,
+  options?: UseFormattedISODateOptions | undefined,
+): ComputedRef<string | undefined>
+export function useFormattedISODate(
+  date: Ref<Date | null | undefined>,
+  defaultValue?: MaybeRefOrGetter<Date> | undefined,
+  options?: UseFormattedISODateOptions | undefined,
+) {
+  const offsetPattern = /(Z|\+\d\d:\d\d)$/
+  function maybeStripOffset(isoDate: string) {
+    return options?.stripOffset ? isoDate.replace(offsetPattern, '') : isoDate
+  }
+
+  function maybeAddOffset(isoDate: string, currentDate: Date | undefined) {
+    if (!options?.stripOffset) return isoDate
+    const match = isoDate.match(offsetPattern)
+    if (match) return isoDate
+    const _currentDate = currentDate ?? new Date()
+    const offset = (formatISO(_currentDate).match(offsetPattern) as RegExpMatchArray)[0]
+    return isoDate + offset
+  }
+
   return computed({
     get() {
-      return formatISO(date.value, { representation: 'date' })
+      return date.value
+        ? maybeStripOffset(
+            formatISO(date.value, { representation: options?.representation ?? 'complete' }),
+          )
+        : null
     },
-    set(dateValue: string) {
-      date.value = parseISO(dateValue)
+    set(dateValue: string | null | undefined) {
+      const _defaultValue = toValue(defaultValue)
+      const offsetRefDate = date.value ?? _defaultValue
+      date.value = dateValue
+        ? parseISO(maybeAddOffset(dateValue, offsetRefDate))
+        : _defaultValue ?? null
     },
   })
 }