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how to convert javascript date according to a new timezone

I thought I had a handle on this, but I cant work it out.
scenario:
1) user selects a date widget which passes back a date in local timezone, lets say 10am 'Australia/Sydney'
2) user then selects a timezone that is different, by identifier 'Australia/Brisbane' (this is a different TZ and may have daylight saving etc...) lets assume its +1hr
What I want to do is have a function that takes a Date object that represents [10am 'Australia/Sydney'] and return to me a new Date that represents [10am 'Australia/Brisbane] i.e. the underlying UTC time will have shifted +1hr
function convertToTimezone(date, newTimezone) {
... what goes here? ...
return newDate;
}
Ive been mucking about with moment timezone and I cant get it to do what I want.

The moment-timezone library should make this trivial:
function convertToTimezone(date, newTimezone) {
return moment(date).tz(newTimezone);
}
Or if date is already a moment:
function convertToTimezone(date, newTimezone) {
return date.clone().tz(newTimezone);
}
See the documentation on Converting to Zone for more information.

OK, FWIW, I got an answer myself. moment.tz doesnt work as I imagined.
To summarise, I want to take a javascript Date that has a wallclock time, say '10am on the 15 sep 2018' that has been associated with a certain timezone identifier, say BrisabneOz.
And turn it into a new date that represents that same wallclock time, but in a different timezone. In otherwords, change the underlying UTC time by the amount required by the shift in timezones and/or daylight savings etc...
The way I found to do this was to get the string of the wallclock date, thus stripping any associated timezone from tbe equation, and using moment.tz to make another date object using the new different timezone. Which it can do.
The part that confused me was having to go to a string as a step - thought I could just pass in one date and get moment.tz to magic me up another date ala #Alex Taylor answer, but this doesnt actually work.
function convertDateToTimezone(date, timezone) {
const str = moment(date).format('YYYY-MM-DD HH:mm:ss');
const tzMoment = moment.tz(str, timezone.identifier)
return tzMoment.toDate()
}

Date Handling Unix Date Incorrectly (or I'm using Date incorrectly?)

I have the following data structure. The first column is intervals. The first row of the interval datum is a unix time and the subsequent data are intervals (i.e. 300*1, 300*2, ect). The other column is the data values. Here is the head of the data:
a1521207300,555.45
1,554.53
2,554.07
3,553.9
4,552.67
And here I went about converting the unix time to a Date object. The a here is ornamental, so I slice() at 1 like so:
var rawTime = data[0].interval;
var timeValue = Math.round(rawTime.slice(1));
console.log(timeValue)
console.log(new Date(timeValue))
I also tried using parseInt() instead of round(). The console shows that this unix time is equivalent to: Jan 18 1970 which I had quite the guffaw at. Then I got to thinking, maybe I did something wrong. It's supposed to be a very recent date -- March 16th 2018. This is strange because my understanding is that javascript can be passed a unix date directly as per this answer.
I also checked the unix time at a conversion site: www.onlineconversion.com/unix_time.htm
Which confirmed that it's indeed a March 16th 2018 timestamp.
Question: Why is this unix date for my March 2018 data being treated like a 1970's date? Maybe the a is actually doing something after all... Anyway, what is the correct way to handle this time stamp? It's only 10 numerical digits, it does not seem to be a precision problem. Date can handle unix times up to 13 digits I believe.

As per the documentation, when you invoke new Date(value) with an integer value, it is used as the number of milliseconds since January 1, 1970. To get the date you want, the value 1521207300 appears to be number of seconds instead of milliseconds. That is, you missed a factor of 1000. new Date(1521207300000) gives Fri Mar 16 2018.
When I take away new from new Date it seems to be ok. Not sure why though.
The documentation mentions the different behavior:
Note: JavaScript Date objects can only be instantiated by calling JavaScript Date as a constructor: calling it as a regular function (i.e. without the new operator) will return a string rather than a Date object; unlike other JavaScript object types, JavaScript Date objects have no literal syntax.
It seems when called as a function Date(value), it treats the value as the number of seconds, instead of milliseconds. I didn't dig deep enough to confirm this, because it doesn't matter: the documentation says to not use it this way (and since it gives a string instead of a date object, it's not very useful anyway).

How to change only timezone without modifying the time in momentz

I have date and time in 2016-06-21T10:00:00-07:00 format which represets 06/21/2016 5 PM in PST, I just want to change this to 06/21/2016 5 PM in EST and vice versa. How can I do it with momentz?
JSFiddle
debugger;
var dateTime = moment('2016-06-21T10:00:00-07:00');
var newDateTime = dateTime.clone();
newDateTime.tz('US/Eastern');
//dateTime = dateTime.utc();
console.log(dateTime.utcOffset());
console.log(newDateTime.utcOffset());
console.log(newDateTime.utcOffset() - dateTime.utcOffset());
//console.log(utc.format());
dateTime = dateTime.add(newDateTime.utcOffset(), 'minutes');
console.log(dateTime.format());
console.log(new Date(Date.parse(dateTime.format())).toJSON());
EDIT:
given input = 2016-06-21T08:00:00-07:00 (PST)
expected output = 2016-06-21T08:00:00-04:00 (EST)
So when I convert that to UTC then it should become
2016-06-22T15:00:00Z for PST
2016-06-22T12:00:00Z for EST

I think you are confused about how ISO8601 format works. This format always represents local time with a time zone offset. Thus 2016-06-21T10:00:00-07:00 represents June 21 2016 at 10 AM in a timezone that is currently UTC-7 (this could be US pacific, among many others).
It sounds like you want to take the local time, but put it in a new timezone. This opens up some interesting questions about why you are receiving the date in the format that you are. If the date is meant to be interpreted as an exact point on the global timeline, then the format you are receiving it in is good. If however, the date is meant to be interpreted as a local time (not relative to UTC), it might be worth considering the possibility that the format of the date needs to be changed at the source. For instance, if you are making an ajax request to an API, and it is returning a date in this format, but that date actually has no relationship to UTC, it would be good to try to change that API to only send the local time (without the offset). If you were able to do that, then the following code would work:
moment.tz('2016-06-21T10:00:00', 'America/New_York').format()
"2016-06-21T10:00:00-04:00"
If you are unable to do that, or if the date is meant to be interpreted as an exact point on the global timeline, but you wish to ignore that in your specific use case, that can be done. You will need to specify a parse format that ignores the timezone offset on your initial time stamp. The code would be as follows:
moment.tz('2016-06-21T10:00:00-07:00', 'YYYY-MM-DDTHH:mm:ss', 'America/New_York').format()
"2016-06-21T10:00:00-04:00"
You might benefit from the material in this blog post, as it covers how ISO8601 format works, and how all of moment's constructor functions work.

Checkout moment().utcOffset() You can pass in the offset as parameter to this function and the date would use that locale.
Assuming you know beforehand the utcOffsets required which in your case are -420 and -240 or -300(EST with DayLightSaving). Below can be done
var dateTime = moment('2016-06-21T10:00:00-07:00');
dateTime.utcOffset(-420).format();
"2016-06-21T10:00:00-07:00"
dateTime.utcOffset(-240).format()
"2016-06-21T13:00:00-04:00"
NOTE: With -04:00, it should 13:00:00 and not 07:00:00 - http://www.timeanddate.com/time/zones/est
EDIT: This answer was posted to the earlier version of question, where same time was needed in different timezones. If it is incorrect, kindly please elaborate on how it is.
Thanks!

Using Moment.js like PHP's date and strtotime

I'm a typically server side developer feeling a bit like a fish out of water trying to display time values on the front end. How can I get behavior like PHP's date() and strtotime() functions out of moment.js? I just want a unix timestamp to appear in H:i:s format, and vice versa.
So far I've tried the following, from existing example code and the documentation:
moment(timestamp).format(H:i:s);
moment().duration(timestamp).format(H:i:s);
moment.unix(timestamp).format(h:mm:ss);
moment(formatted,'H:i:s');
Not a SINGLE one of which has worked properly. This may get flagged as duplicate since there are plenty of moment.js questions out there, but I don't know whether it's been updates to the library itself or slightly different context, I have not found one existing solution that has worked for me.
Anybody have any suggestions for these two simple tasks?
EDIT:
I've distilled two different problems out of this. One is that functions the moment docs say should work are giving weird values:
moment(1437462000).format('h:mm:ss')
for instance, which should return 7:00:00 utc, returns 10:17:42. This can be fixed in this case by using moment.unix(1437462000).utc().format('h:mm:ss') instead, but this leads into the second problem - the .utc() function seems to get ignored when converting back from a date into a timestamp:
timestamp = moment(formatted,'DD/MM/YYYY H:m:s').utc().unix();
will still return a timezone corrected value (in my case this is incorrect by several hours since the formatted time in question has nothing to do with the client computer) regardless of whether the .utc() function is included or not.

A few things you should realize:
Unix timestamps should always in terms of UTC. They are never adjusted for time zone in numerical form. If they're adjusted for time zone, that's done during the interpretation of the number, not in its representation.
While traditionally a "Unix Timestamp" is in terms of seconds, many environments use milliseconds instead. PHP's date timestamps are based on seconds, while moment and JavaScript's Date object both use milliseconds by default. Using the moment.unix function will let you pass seconds, and is identical to just multiplying the timestamp by 1000.
Moment has two built-in modes, local and UTC. The default mode is local. It doesn't matter what input you provide, if you don't specify UTC, the moment is adjusted to local. To specify UTC, you use the utc function. There are two forms of the function:
moment.utc(input) // parsing form
moment(input).utc() // conversion form
Both forms take some input and result in a moment in UTC mode. The difference is in how the input is interpreted. In either case, if the input value is unambiguous, the result is the same. For strings, that means the input would contain either a Z (from ISO8601), or a UTC-based offset. All other forms are ambiguous. For example, if I pass "2015-11-08 01:23:45", I will get different results depending on whether I interpret that string as local time or as UTC.
For numbers, they are always interpreted as milliseconds in UTC. However, if you use moment(number) without then calling .utc() then the moment is left in local mode, so any output will display as local time.
When you call moment.unix(input), the input is a number of seconds, but the moment is left in local mode. So to display the UTC time, you would use moment.unix(input).utc().
If your pre-recorded timestamps from your other system are in numeric form, but have been adjusted away from UTC, then they are incorrect. You have bad data, and Moment can't help you unless you know specifically how they have deviated and you write code to counteract that.
Moment's formatters are case sensitive. M is months, m is minutes. H is hours on a 24-hour clock, h is hours on a 12-hour clock. Use two consecutive letters when you want to include zero-padding. Example, HH:mm:ss for 13:02:03 vs. h:m:s for 1:2:3.
Moment's X formatter does not care which mode the moment is in. It will always emit seconds in UTC. Likewise, the x formatter returns milliseconds in UTC, as does moment.valueOf().
Also, your last example:
moment.unix(1437462000).utc().format()
Returns "2015-07-21T07:00:00+00:00" - which I believe is the value you expected.
You also get the same original timestamp regardless of which of these you try:
moment.unix(1437462000).utc().format("X") // "1437462000"
moment.unix(1437462000).format("X") // "1437462000"
moment.unix(1437462000).utc().unix() // 1437462000
moment.unix(1437462000).unix() // 1437462000

For anyone who comes in and is still looking for direct PHP equivalents for date() and strtotime(), here are the ones I ended up using. Matching up to php basically means just completely ignoring any kind of local time information by making sure everything is in UTC. That task is a little different between the timestamp->date and date->timestamp cases, though, so you have to be careful.
date()
Converting a timestamp to formatted date without any client timezone correction
var formatted = moment.unix(timestamp).utc().format('h:mm:ss');
strtotime()
Converting a UTC formatted date back to a timestamp without correcting it to local time:
var new_timestamp = moment.utc(formatted_utc,'DD/MM/YYYY H:m:s').format('X')
//where 'DD/MM/YYYY H:m:s' is the formatted date's format, and
//'X' outputs a unix timestamp without milliseconds.
Notes:
Do not use moment() with parenthesis in the calls:
moment().utc(date,format) will return local time values, not your
input.
Moment.js does not like the use of 'i' for minutes in the formatting,
unlike php.

momentjs: how to get the date in a specific timezone

In a nutshell I want moment to respect server's timezone. I've set my machine's timezone to Alaska but I'm passing a Brisbane timezone string to moment. Now I need moment.toDate to return a date instance in the same timezone as the one I pass in the moment constructor; e.g.
m = moment("2016-11-20T08:00:00+10:00")
m.format() // "2016-11-20T08:00:00+10:00"
m.toDate() // Sat Nov 19 2016 13:00:00 GMT-0900 (AKST)
I want to get a Date instance from moment that's in the input timezone; e.g. somehow get toDate to return Sun Nov 20 2016 08:00:00 GMT+1000 (AEST).
FWIW I have tried the same code with and without moment.tz.setDefault and while it correctly changes the format result, toDate always uses the machine's timezone!
Update
The reason I need this behaviour is that some JavaScript libraries and controls don't understand moment and only work with Date and the time/date gets skewed when presented back by them. One example, the one I'm currently dealing with, is jQuery UI date picker control. I want the date picker to show the current date as it's on the server (or on a specific timezone).
Thanks in advance.

The Date object represents the time in UTC internally, and can only use the time zone of the machine its running on when projected.
There's absolutely no way to produce a Date object that uses an arbitrary time zone. Any examples you may come across that try to manipulate the Date object (such by adding or subtracting time zone offsets) are fundamentally flawed.
Moment itself has great time zone support, including the moment-timezone extension for working with named time zones instead of just time zone offsets. But once you go back to a Date object - you're back at the mercy of the behavior of that object.
Sorry, but there's no way to achieve what you are asking. Perhaps you could elaborate as to why you wanted to do this, and I could recommend a workaround.
Update: With regards to your update, usually there is a mechanism for getting the value from a date picker as text, rather than as a date object. With the example of the jQuery UI date picker control, the onSelect event gives it to you as text already, or you can simply call .val() instead of .datepicker('getDate') to get the text out of the field. Once you have a textual value, you can then parse it with moment however you like.
Similarly, when setting the value, you don't necessarily need a Date object. You could just set the value of the textbox as a string, or you can pass a string to the setDate function.
In most cases, you won't have to go through a Date object. But if for some reason you do, then you'll need to artificially construct one with something crazy like:
var d = new Date(m.format('YYYY/MM/DD'));
Normally, I'd be against that - but if it's just there to get the pass a value to a UI control, then it's probably ok.

This will get you a moment in the same timezone as the moment string, but toDate is always in the local timezone.
d = "2016-11-20T08:00:00+10:00"
m = moment(d).utcOffset(d)
m.format()
m.toDate()