Javascript Date

So I need a way to represent LocalDate that can be found in java.time package. Turns out it’s harder than it looks.

Take LocalDateTime for example: LocalDateTime is a relative date that’s the same no matter which time zone you’re in. The easiest thing is to just use string: 2020-07-25T01:02:03. Note that there is no zone/offset information in the string

But if you want to use a Date object in javascript to represent it, it gets very tricky.

My first attempt is to just subtract the users’ zone offset like so:

const offsetInMillis = new Date().getTimezoneOffset() * 60000 // minutes to ms

function toLocalDatetime(date: Date|number): Date {
  let relativeTimestamp;
  if (date instanceof Date) {
	relativeTimestamp = date.getTime() - offsetInMillis
  } else {
    relativeTimestamp = date - offsetInMillis
  }
  return new Date(relativeTimestamp)
}

I thought naturally if you create a date from string without timezone, it would create it in the user’s local timezone. But actually it’s not, see MDN.

❯ node
Welcome to Node.js v17.5.0.
Type ".help" for more information.
> Date.parse('01 Jan 1970 00:00:00 GMT')
0
> Date.parse('01 Jan 1970 00:00:00')
-3600000
> new Date().getTimezoneOffset()
0

Conclusion

Bottom line is using just Date is not enough to represent LocalDateTime. If you’re in the same situation, just use js-jodatime