Subtracting two DateTimes to return a Duration?

This was spurred by my desire to be able to write in RSpec something like:

expect(a_datetime).to be_within 5.minutes of some_other_datetime

All that would be needed to make this and other such comparisons work (as far as I can tell) is to implement the ‘minus’ method such that subtracting two DateTimes returns a Duration.

Seems like it might make a nice addition.

Opened #27436 for comment.