"Unix timestamp to Excel date: why serial 60 is a day that never existed"

You paste 1756771200 into a spreadsheet and want to see a date. Or you export a sheet and your API receives a column of numbers like 46236.5 instead of dates. Both directions look trivial and both go wrong in the same three places: the epoch, the unit, and one fake day in February 1900. What the two numbers actually count A Unix timestamp is the...