opaque type example (DateInTimezone)

index.ts

declare const _date: unique symbol
export type DateInTimezone = Date & { readonly [_date]: Date }

/**
 * Checks if the date is between 12am and 4am
 * of the day
 *
 * Returns true if it is, false otherwise
 **/
export function isBetween12amAnd4am(date: DateInTimezone) {
  const hours = date.getHours()
  return hours >= 0 && hours < 4
}