how to extract the local time and year when given a UTC time in milliseconds and the timezoneOffset?

Question

In javascript, one is given:

1. UTC milliseconds since 1970
2. timezoneOffset of the local timezone

From the above, how to extract the local time and year implied by the timezoneOffset?

Answer 1

your question is quite ambiguous, but...

Extracting the exact local time from just UTC milliseconds and timezoneOffset in JavaScript isn't entirely possible. Because TimezoneOffset only provides Offset and Some timezones have DST changes throughout the year.

You can still do something like that:

function getLocalTimeFromUTC(utcMillis, timezoneOffset) {
  // 1. Create a Date object from UTC milliseconds
  const date = new Date(utcMillis);

  // 2. Adjust for timezone offset (in minutes)
  date.setMinutes(date.getMinutes() + timezoneOffset);

  // 3. Extract local year
  const year = date.getFullYear();

  // 4. Local time with potential DST ambiguity (consider libraries for advanced handling)
  const localTime = date.toLocaleTimeString();

  return { year, localTime };
}