Comment by maxiepoo

I think the concept you are looking for is a ["torsor"](https://en.wikipedia.org/wiki/Principal_homogeneous_space).

Basically,

1. You have a 0 time delta, and you can add and subtract them satisfying some natural equations. (time deltas form a group)

2. You can add time deltas to a datetime to get a new datetime, and this satisfies some natural equations relating to adding time deltas to each other (time deltas act on datetimes).

3. You can subtract two datetimes to get a time delta satisfying some more natural equations (the action is free and transitive).