Get timezone difference in hours with moment

Question

I would like to get the timezone difference between New York and Hong Kong with Node.js moment module. I have done some preliminary work.

var NewYork_time_hr = moment().tz("America/New_York").format('HH'); 
var HongKong_time_hr = moment().tz("Asia/Hong_Kong").format('HH');

I can then proceed to write a function to calculate the difference between the 2 timezones in hours. I was hoping for a simpler method.

Is there a more elegant and simpler way to do it with moment library?

Solution

Not sure about "simpler", but more correct (since not all timezones are a full hour from each other):

// get the current time so we know which offset to take (DST is such bullkitten)
var now = moment.utc();
// get the zone offsets for this time, in minutes
var NewYork_tz_offset = moment.tz.zone("America/New_York").offset(now); 
var HongKong_tz_offset = moment.tz.zone("Asia/Hong_Kong").offset(now);
// calculate the difference in hours
console.log((NewYork_tz_offset - HongKong_tz_offset) / 60);

<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.10.6/moment.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment-timezone/0.4.1/moment-timezone-with-data-2010-2020.min.js"></script>

Comments

[guagay_wk]

Upvoted and selected as answer. I don't quite understand why the need for var now = moment.utc();? Why not just get NewYork_time_hr offset of HongKong_time_hr?

[Amadan]

The comment above it was a hint - timezone offsets change throughout the year, and not all at the same time. The difference between Hong Kong (has no DST) and New York (has DST) is different depending on whether you're asking in summer or in winter. And you can't just use hr because, e.g. Calcutta is offset at +05:30.

[Matt Johnson-Pint]

Great answer. Though it will also work if you just pass moment() instead of moment.utc(). It still gets evaluated as the same UTC-based timestamp regardless of which form you pass. +1 about the result being different depending on when you call it. "How many hours between New York and Los Angeles?" (Usually 3, but sometimes 2 and sometimes 4.)

[Matt Johnson-Pint]

FYI, the other way to obtain an offset is moment(now).tz("America/New_York").utcOffset(), but either way is fine. You still have to subtract them manually.