date currently accepts anything, tries to convert input to a date, and if it fails, then assumes it is something like "1" or "1,5" for DOY.

I think it should:

• accept Dates, character values that can be converted to dates, or integer-ish vectors.
• give an informative error if it is anything but those

This means deprecating date = "1,5" in favor of date = c(1,5) and shoring up the date parsing and formatting since as.Date(c(1,5)) doesn't error