Negative year date functions update

[danrademacher]

This is for OpenHistoricalMap/issues#634

[danrademacher]

@Rub21 this is the PR with the updated date functions.

The change in teh date functions themselves is in this commit. It's a bit convoluted, so I would say we deploy this to staging, make sure the database is using it, then GreenInfo can test on staging with some specific cases to be sure it is working as expected.

So I think the main work here for you would be to make sure we can repopulate the decimaldate in the tiles based on these revised functions once they are merged on staging. That happens with the isodatetodecimaldate query in the TOML files: https://github.com/search?q=repo%3AOpenHistoricalMap%2Fohm-deploy%20isodatetodecimaldate&type=code