Best way to calculate date and time?

IIRC in ClarionMag you are looking for “StarDate” or “StarDates” by Dr Parker for the Date.Time format.

Unix Dates are “Number for Seconds since 1/1/1970”. It can and fit into a LONG for something like 20 more years, but a ULONG for something like 90 so past 2100. You could use any base date.