Enforce UTC for timestamps

[jaburgoyne]

So far as I can see, the timestamps in our JSON exports have always been and are still in the UTC time zone. For consistency, I fully support this choice!

I'm not sure, however, that we formally enforce this anywhere or have it documented. Could we do that?

[drikusroor]

The configured time zone for MUSCLE is Europe/Amsterdam. However, the settings USE_TZ is also turned on, which means that all datetime objects are stored timezone-aware.

[jaburgoyne]

That sounds good: I would already perhaps document somewhere that we always want this to be so (and also include it in the documentation for setting up MUSCLE elsewhere).

The issue also has to do with JSON exports. It seems like – probably because of USE_TZ – the export defaults to UTC. I like that! But let's make it explicit rather than relying on a default (if we haven't done so already).