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.