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 number of seconds since 1 January 1970, 00:00 UTC. Leap seconds are ignored, so every day is exactly 86,400 seconds long. A timestamp from today has 10 digits. If you see 13 digits it is milliseconds (what Date.now() returns in JavaScript). 16 digits is microseconds, 19 digits is nanoseconds (Go, Rust and several databases use those).
An Excel serial date is the number of days since Excel's epoch, with the fraction of the day as the decimal part. 46236.5 is noon on the day with serial 46236. Excel never stores a time zone; the number is whatever you typed.
The formulas
