Well, numpy's datetime64 can be set to use (almost) whatever unit you want:

 

Though it uses a single epoch, which I don't think ever made sense with femtoseconds....

And it has other problems, but it was designed that way, just for the reason.

However, while there has been discussion of improvements, like making the epoch settable, none of them have happened, which makes me think that no one is using it for physics experiments, but rather plain old human calendar time...

-CHB