Please add description of "since" parameter on projects and tasks to the API docs

The hourlyRate property on projects and tasks has a “since” parameter that needs explaining in the documentation. Poking with some values reveals this:

{“message”:“You entered invalid value for field : [since]. Values that represent [since] can be null and can’t be empty and can’t be in future. Please make sure that the [since] date is not greater than 9999-12-31 and not less than 0001-01-01. Ensure that the [since] date is in following format: “yyyy-MM-ddThh:mm:ssZ” Example: 2018-11-29T13:00:46Z.”,“code”:501}%

However it is not obvious if there is a special value to indicate “ALL” time entries or do you simply specify the 0 date?