/**
 * Converts a datetime-local input value (local time, no timezone info)
 * to a UTC ISO string for server submission.
 *
 * `datetime-local` inputs use the format "YYYY-MM-DDTHH:MM" and do NOT accept
 * ISO strings with a timezone designator (e.g. "2026-03-11T17:00:00.000Z").
 * Setting an ISO UTC string directly on a datetime-local input causes the browser
 * to silently clear the field value. Use this function with a separate hidden input
 * to hold the converted UTC value before form submission.
 *
 * Returns null when value is empty, nullish, or not a valid date.
 *
 * NOTE: The returned ISO string (e.g. "2026-03-11T17:00:00.000Z") is intentionally
 * incompatible with datetime-local inputs, which only accept "YYYY-MM-DDTHH:MM".
 * Setting a Z-suffixed string on a datetime-local input causes browsers to silently
 * clear the field. Always write the result to a separate hidden input, not back to
 * the datetime-local input itself.
 */
export function localDateTimeToUtcIso(value: string | null | undefined): string | null {
  if (!value) return null;
  const date = new Date(value);
  if (isNaN(date.getTime())) return null;
  return date.toISOString();
}