Thomas Beutlich

I started with an operator record for TimeType to also allow addition, subtraction etc. That meant, we also would need to consider a TimeDeltaType and conversion to absolute time, which...

[Here](https://gist.github.com/beutlich/f090c7a010b109fa1418495f64ff2705) is a first (and incomplete) draft for conversion of date/time to string. Not sure if it is good enough ❔ . Any feedback is appreciated.

@HansOlsson @DagBruck @t-sommer @StephanZiegler Yesterday, I noticed by chance that there are Testing.Utilities.Time.{DateTime, Duration} available in Dymola as operator records. This is exactly the way I wanted to go here...

@m-kessler Thank you for the generous offer. I rebased the commits of this PR on master and resolved the merge conficts. This PR branch is now ready to be used....

> @beutlich shall we merge it? Sure, please go ahead. Push directly to this PR branch as you already did. I'll answer the other issues you observed later.

> > > The new external function ModelicaTime_strftime is nice and definitely the way to go (using the existing C functions to format time, instead of rewriting it in Modelica)....

> It should -2d -5min, right? Correct. Fixed by ca583ce273096856d65c7964b216d33c0bb375eb.

Hm, I only looked at section [7.1.3](https://specification.modelica.org/v3.4/Ch7.html#restrictions-on-the-kind-of-base-class) where it reads: > The specialized classes [...] operator record [...] can only be derived from their own kind [...] and from class....

> > We could make it a class then. Also works in SimulationX without warnings.

* I spent many hours to work around the 1970 restriction that is in current code. The solution is not yet ready and not that nice. My proposal would be...