An Explanation of UTC TIME SEC by Dave Bruns of excelijet along with a quick video on the N2
The Unix time stamp tracks time as a running count of seconds. The count begins at the "Unix Epoch" on January 1st, 1970, so a Unix time stamp is simply the total seconds between any given date and the Unix Epoch. Since a day contains 86400 seconds (24 hours x 60 minutes x 60 seconds), conversion to Excel time can be done by dividing days by 86400 and adding the date value for January 1st, 1970.