There are many sites out there that will give you a full description of what the UNIX Epoch standard date system is, why it is used, and a thorough understanding of why that’s important to you. You also probably don’t have time for a history/computer science course when all you want to do is convert that time stamp into something that looks nice for your boss in Excel. In that case, here you go:

All examples shown assume that the UNIX time stamp is in column A.

GMT – Greenwich Mean Time – This is the standard example you see across the Internet
=(A1/86400)+25569

EST – Eastern Standard Time (GMT – 5:00)
=((A1-18000)/86400)+25569

CST – Central Standard Time (GMT – 6:00)
=((A1-21600)/86400)+25569

MST – Mountain Standard Time (GMT – 7:00)
=((A1-25200)/86400)+25569

PST – Pacific Standard Time (GMT – 8:00)
=((A1-28800)/86400)+25569

Now set your column format to a Date field to your liking. I use the 3/14/01 13:30 format so I can include the easily sortable military time format.