Make time tests time-zone-independent

[jeremyruppel]

@kagd added a test for 0600 in #28, but it would be great if we didn't have to deal with that BS.

[kagd]

We can probably just convert the time that is being passed to UTC time since that will remain constant.

[jeremyruppel]

The tricky part with this one is that we typically don't want to display UTC to the user; it'll more likely be in their local time zone. So, what would be nice is to always have the test suite run in a specific time zone (doesn't matter which one).

I was thinking something like https://github.com/vesln/timekeeper might be of use here. Freeze time in a certain zone before the time-dependent tests.

[kagd]

Good point. I will try that out and see if I can get something working.