How do I launch a process with different date/time settings than the rest of the system? I'm developing a desktop app for OS X with behaviour that depends on the date. I want to test the behaviour without waiting for a specific date.

I'd rather not change the date settings of the whole system as I'm worried it might mess with build systems etc. Using a VM is possible, but takes a while to set up.

Is there a simple way to run a process and its children in a time delta relative to the actual time, without affecting the rest of the system?
No. The system doesn't bifurcate time.

You would need the app in question to handle the time shift or run independently of the system time or package the app to run in a VM or other container like docker. In some limited cases, the time zone variable might help you make a little tweak, but arbitrarily setting a totally unrelated time isn't implemented on OS X.