Calendar: Option to start months & days at 0

I’m working with a fictional calendar system that starts its months and days at 0 rather than 1; I’ve noticed there are options for customizing the number of months, month names, and the number of days in each month, but couldn’t find anything that would allow me to do this. I can work around this by offsetting each month and day by 1, but it is a bit inconvenient having to write (for example) what should be 00-00-1000 as 01-01-1000, or 04-04-1000 as 05-05-1000, and convert every time I need to write something down.

Apologies if there is an option or a request topic for this already. I looked around and didn’t find anything.

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Years starting at 0 would be useful as well!
I had asked if it was possible in the past, but never got around to a formal request.