Simplify date/time operations with additional helper functions

[nonprofittechy]

Date math with format_date is a little confusing with the requirement to convert the Docassemble built-in date type and the function name "format_date" doesn't make it clear you can use it for date math. EDIT: I see now that the format_date part is not needed.

1. Get a date in the interview variable case.answer_date, and add 2 weeks to it:

format_date(as_datetime(case.answer_date) + date_interval(days=14))

With example 1, you need to change the format of the date in the interview into a datetime. Otherwise you get an error that is confusing for a novice like me:

Error
TypeError: coercing to Unicode: need string or buffer, relativedelta found

I tested it with format_date(today() + date_interval(days=14)) which worked, so today() appears to a return a datetime while Docassemble datatype: date does not.

I suggest adding wrapper functions for add_days, add_months, etc, or alternatively, add_dateinterval that accepts the same arguments as date interval. It should work without requiring the interview author to convert a Docassemble date to a datetime. I think for legal applications, add_days would be most common though? Perhaps also add_years?

E.g.: add_days(case.answer_date, 14) or add_dateinterval(case.answer_date, days=14). It should accept both the Docassemble date datatype and the datatype of the today() function.

[nonprofittechy]

Alternatively--if it's not a breaking change, datatype: date could default to a datetime object instead of the current format, so it works with the existing examples?

[nonprofittechy]

Resolved with commit 4b722c1