Convert UTC Epoch to local date

I have been fighting with this for a bit now. I’m trying to convert epoch to a date object. The epoch is sent to me in UTC. Whenever you pass new Date() an epoch, it assumes it’s local epoch. I tried creating a UTC object, then using setTime() to adjust it to the proper epoch, but the only method that seems useful is toUTCString() and strings don’t help me. If I pass that string into a new date, it should notice that it’s UTC, but it doesn’t.

new Date( new Date().toUTCString() ).toLocaleString()

My next attempt was to try to get the difference between local current epoch and UTC current epoch, but I wasn’t able to get that either.

new Date( new Date().toUTCString() ).getTime() - new Date().getTime()

It’s only giving me very small differences, under 1000, which is in milliseconds.

Any suggestions?

I think I have a simpler solution -- set the initial date to the epoch and add UTC units. Say you have a UTC epoch var stored in seconds. How about 1234567890. To convert that to a proper date in the local time zone:

var utcSeconds = 1234567890;
var d = new Date(0); // The 0 there is the key, which sets the date to the epoch
d.setUTCSeconds(utcSeconds);

d is now a date (in my time zone) set to Fri Feb 13 2009 18:31:30 GMT-0500 (EST)

It's easy, new Date() just takes milliseconds, e.g.

new Date(1394104654000)
> Thu Mar 06 2014 06:17:34 GMT-0500 (EST)

And just for the logs, I did this using Moment.js library, which I was using for formatting anyway.

moment.utc(1234567890000).local()
>Fri Feb 13 2009 19:01:30 GMT-0430 (VET)

Epoch time is in seconds from Jan. 1, 1970. date.getTime() returns milliseconds from Jan. 1, 1970, so.. if you have an epoch timestamp, convert it to a javascript timestamp by multiplying by 1000.

   function epochToJsDate(ts){
        // ts = epoch timestamp
        // returns date obj
        return new Date(ts*1000);
   }

   function jsDateToEpoch(d){
        // d = javascript date obj
        // returns epoch timestamp
        return (d.getTime()-d.getMilliseconds())/1000;
   }