Aside from the Gregorian Calendar System there is a widely used Persian Calendar System also known as the Jalali Calendar System which Coda does not support. This doc has a simple solution for supporting Persian Calendar System alongside the native date.
It simply includes a table of 366 records (equivalent to the amount of days in a leap year), named
with auto generated columns (all formulas) mimicking the current Persian Calendar year (
Since its a simple Coda table, you can easily convert any Gregorian Calendar Date to any part of its equivalent Persian Calendar Date (or vice versa), using lookup or filter.