ex_cldr_calendars also provides the ability to add date parts to dates: Cldr.Calendar — Cldr Calendars v2.4.4

    iex> Cldr.Calendar.minus ~D[2019-03-01], :days, 1
    ~D[2019-02-28]
    
    iex> Cldr.Calendar.minus ~D[2019-03-01], :months, 1
    ~D[2019-02-01]
    
    iex> Cldr.Calendar.minus ~D[2019-03-01], :quarters, 1
    ~D[2018-12-01]
    
    iex> Cldr.Calendar.minus ~D[2019-03-01], :years, 1
    ~D[2018-03-01]

@josevalim, there’s no CLDR dependency on this part - its all straight Calendar callbacks.

The only part that is tricky is what to do with something like Calendar.shift ~[2016-02-29], :year, 1. It is an invalid result?

In my implementation I have an option :coerce to force a valid date at the end of the target month. I’ve never liked it. Suggestions very welcome on how to handle this.

Happy to draft a PR for this, its not very complicated other than that issue.

I would love to have this feature in Elixir. To me it is really the last pending feature in the Calendar module. It seems it is a simple contract on a function called “plus”, which I would call “shift” instead.

@kip, how are the rules generated in your case? Automatically from CLDR? Is there a way we could see the rules converted to Elixir code for the ISO calendar?

Thank you!