TimesDates.jl dev for v2

[jar1]

The significance of milliseconds for the DateTime API has been debated recently on here and GitHub.

Dates: Documentation, tests, and changed arithmetics for `DateTime`

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

There is currently a PR on the repo with very lively discussion about what the following operation should do: using Dates a = now() b = a + Nanosecond(1) (the situation is analogous with Microsecond) Currently, the result is always exactly equal to a, because the DateTime arithmetic implemented in Dates simply truncates the non-DateTime argument to milliseconds (as that is the supported precision of DateTime), and whatever the result is, will be added to DateTime: julia> using Dates julia>…