Consider returning a process' creation time as a SystemTime

[HadrienG2]

Since you return an elapsed time since the Unix epoch, you already did 95% of the work required to use SystemTime here, and SystemTime is...

1. Closer to std and chrono conventions.
2. More convenient to use for creation/modification date purposes.

[svartalf]

I like this idea, because in a current implementation not all used unit types are semantically that good to represent what values they are carrying.