`DateTime` arithmetic on `Microsecond` (or smaller) scale

[Sukera]

@anowacki

Firstly, my opinion is that any change here apart from documentation is breaking. That includes the proposal in the PR to change the rounding mode from floor to round. I do not believe that should happen without a major version change (presumably if/when Dates becomes an upgradable stdlib), because it would change the behaviour of people’s programs.

Would you be alright with a deprecation of the arithmetic between DateTime and Microsecond/Nanosecond? As you say, we can’t just make it error, because that breaks working code. A deprecation notice should be ok though.