/**
 * Convert the most common types of dates to the ISO8601/RFC3339 format,
 * in the local time zone, without milliseconds.
 *
 * @param {Date|number|string} [d = Date.now()] - the date, either a Date object,
 *   or a UNIX timestamp in (milli/micro/nano)seconds
 * @return {String} ISO8601/RFC3339 `YYYY-MM-DDTHH:MM:SS` string in the local timezone
 */
export function localISOdt(d?: Date | number | string | boolean): string {
  if (!d)
    d = new Date();
  if (/^\d\d\d\d-\d\d-\d\d/.test(d as string)) return d as string;  // leave YYYY-MM-DD strings unchanged
  if (/^\d+\.?\d*$/.test(d as string)) {
    // We have a (possibly fractional) number. Date takes a millisecond argument, so
    if (d < 4102512345) (d as number) *= 1000;  // ...interpret as seconds if before 1/1/2100
    if (d > 4102512345000) (d as number) /= 1000;  // ...and as microseconds if after 1/1/2100
    if (d > 4102512345000) (d as number) /= 1000;  // ...and finally as nanoseconds
  }
  if (!(d instanceof Date))
    d = new Date(d as number);  // parse it
  return new Date(d.getTime() - d.getTimezoneOffset() * 60000)  // the offset is in minutes
    .toISOString().slice(0, -5);  // drop the .milliseconds and 'Z';
}