Taking a lead from @Jeffrey's comment this is how you can calculate the Unix timestamp.

= ( A1 - DATE(1970,1,1) ) * 86400

The reason is datetime values in Google Sheets, Excel and other spreadsheets have an epoch of 30th December 1899 whereas Unix epoch is 1st of Jan 1970. There's a bit of tech history around this: https://stackoverflow.com/a/73505120/2294865

Remember datetime/timestamp value is generally interpreted naively as UTC timezone so be aware of this when converting to/from date & time which typically take on local timezones and daylight savings adjustments.