feat: Add time constants for Day, Week, Month, and Year

[liorkesten]

PR Description

Added time constants for Day, Week, Month, and Year to ensure consistency and simplify access, avoiding inline calculations like 24 * time.Hour.

[samber]

hey @liorkesten

There is a good reason if go does not bring those constants already.

1 day is not always 24 hours
1 month is not always 30 days
1 year is not always 365 days

The stdlib comes with t.AddDate(...), but i think there are some libraries for accurate computations.

Feel free to argue until I close the PR.

[senago]

There's a pretty cool list of common falsehoods about time: https://gist.github.com/timvisee/fcda9bbdff88d45cc9061606b4b923ca