Javascript Date - how to know whether DST is in effect in a given timezone

Question

First off, I am NOT looking for whether DST is in effect locally.

I'm running a Node process that has data that has associated timestamps. I need to convert those timestamps to a day/month/year in a specified time zone, but all I'm given is the time zone's offset and DST offset.

I wish Date / Moment worked better with time zones. They work great with UTC or Local time zones, but it seems you need to hack it to get something else.

Is there something I'm missing? Assuming I can determine whether DST is in effect, would this work:

var d = new Date(ts + timezone_offset - local_offset);
d.getMonth();
d.getDate();

where timezone_offset is the time zone's offset (either the standard offset or the dst one)?

How might I determine whether DST is in effect?

Answer 1

First, recognize that if all you have are the offsets, you cannot solve this problem. You must have a time zone identifier, such as "America/New_York". Since in the question comments you said you do indeed have this, then you can use one of these libraries to get the job done.

I had previously posted another answer to this question, recommending TimeZoneJS, but I'll retract that now - as this question is specifically about DST, and TimeZoneJS has bugs, reporting values incorrectly near DST transitions.

Instead, I'll now recommend using moment.js with the moment-timezone add-on. Once installing both libraries (being sure to load actual time zone data, per the docs), the code is quite simple:

moment(timestamp).tz(timezone).isDST()

For example:

moment(1451624400000).tz('America/New_York').isDST(); // false
moment(1467345600000).tz('America/New_York').isDST(); // true

Or, if your timestamps are already strings in terms of the local time, then use this syntax instead:

moment.tz('2016-01-01T00:00:00','America/New_York').isDST(); // false
moment.tz('2016-07-01T00:00:00','America/New_York').isDST(); // true