How to determine if two IANA time zones are the sa

Question

Given two different IANA (aka "tz database" or "Olson") time zones, how can I determine if they represent the same zone?

For example, Asia/Kolkata and Asia/Calcutta represent the same zone.

Another example, Africa/Asmara and Africa/Asmera represent the same zone.

These minor spelling differences matter when using APIs that detect or guess the user's time zone, as one cannot then simply compare strings to a previously stored value, because different implementations return different variations of the time zone identifier.

Answer 1

Modern JavaScript implementations that fully support the ECMAScript Internationalization API, can use the following:

function areSameTimeZones(zone1, zone2) {
  var resolvedZone1 = Intl.DateTimeFormat(undefined, {timeZone: zone1})
                          .resolvedOptions().timeZone;
  var resolvedZone2 = Intl.DateTimeFormat(undefined, {timeZone: zone2})
                          .resolvedOptions().timeZone;
  return resolvedZone1 === resolvedZone2;
}

For applications requiring compatibility with older browsers, Moment-Timezone can be leveraged:

function areSameTimeZones(zone1, zone2) {
  var z1 = moment.tz.zone(zone1);
  var z2 = moment.tz.zone(zone2);
  return z1.abbrs === z2.abbrs && z1.untils === z2.untils && z1.offsets === z2.offsets;
}