Add support for BYEASTER

[rubengees]

Hello!

I have a feature request: Add support for BYEASTER. This is not in the RFC if I understood correctly, but an extension some implementations provide. Two examples are the python library and the js library.

This would be useful if you want to model certain holidays using rrules.

[dmfs]

I need to think about this. Most importantly how this relates to the the other parts of a RRULE. For instance, I assume this would only be valid in a FREQ=YEARLY rule. I guess this part would always be expanded and never filtered.

That said I wonder if a better (more scalable) implementation would be a new RSCALE type that has special behaviour. I.e. an RSCALE that behaves exactly like a Gregorian Calendar but starts counting the year days on Easter Sunday. I'll think about that too.