Storing dates as numbers in a spreadsheet has been a thing since the first spreadsheet program I know of. Microsoft picked "days since 1900". If you're on UNIX, you may prefer using "1199188800" instead.

Other than that, the difference is pretty minor. ODS is very verbose and stores the content of the cell twice for some reason, but the XML trees are essentially the same.

The best way for corporate interaction is to export to whatever the hell Microsoft Excel accepts as an external data source, because .xslx files can natively import remote data that way. Hope your customers' computers are all configured for en_US mode, though, because CSVs aren't as universal as people pretend they are.

I'm somewhat fine with the unix epoch timestamp, but this thing is not just seconds or microseconds since x, it is days since x, plus the rest of the time bits as a fraction. One second to midnight on January 1st 2008 is… 39448.9999884259.

Oh, and while 39448.5 is fine, 39448.0 makes Excel throw an error and refuse the whole document. Midnight January 1st 2008 is just 39448. The parser cannot handle 39448.0.