Unit testing handling of date inputs in JavaScript regardless of time zone

Unit testing handling of date inputs in JavaScript regardless of time zone

I have a form where a user can enter a date, i.e. <input type="date"> the
value is submitted in yyyy-MM-dd format. When I create a Date object with
the string it assumes the time zone is the one the user's browser is set
to – this is the behavior I want.
I'm then using the date value to make queries against a REST API that
expects ISO date/time strings. That's no problem as the toISOString
function on the Date object handles everything correctly.
However, when I'm unit testing this code – setting my input to a
yyyy-MM-dd string then asserting that the output is an expected ISO
timestamp string the tests can only work in a particular time zone. Is
there a way I can force the time zone in the test?
I've tried using Jasmine spies to do something like:
var fixedTime = moment().zone(60).toDate()
spyOn(window, 'Date').andCallFake(function() {
return fixedTime;
});
But given there are so many variants of the constructor and so many ways
it gets called by moment.js this is pretty impractical and is getting me
into infinite loops.