There are a lot of posts about converting Unix Epoch dates to Excel (which is a number representing the number of days since 1900-01-01 vs. the number of seconds since 1970-01-01) and a few on converting Excel dates to Epoch.

The quick formula for converting an Excel date to an Epoch date is:

(EXCELDATE - 25568) * 60 * 60 * 24

This is slightly modified from a comment to a post that I can't seem to find spur of the moment.