The backend should treat all Dates as UTC

[codeliner]

Extracted from #4. Based on a comment by @prolic
The frontend is required to set the appropriate timezone.
Currently we use "$dateTime = new DateTime()", which relies on php's internal timezone setting. If you query the data from another server with different timezone settings, you get wrong DateTimes for the created_at property of an event.

[prolic]

How would you like to implement this?
We can do it on every single adapter (calculate the offset from UTC and substract) or direct in the DomainMessage class?

[codeliner]

We should fix that in the DomainMessage class and publish a bugfix release.