Aside from the Gregorian Calendar System there is a widely used Persian Calendar System also known as the Jalali Calendar System that Coda does not support. This doc has a simple solution for supporting Persian Calendar System alongside the basic date system.
It simply includes a table of 366 records (equivalent to the amount of days in a leap year), named Jalali Calendar with auto generated columns (all formulas) mimicking the current Persian Calendar year (1389).
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.
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