Assuming your millisecond timestamp is in cell A1

=(A1/1000 + ("1/1/1970"-"1/1/1900"+1)*86400) / 86400

or if you have rows with missing timestamps:

=IF(A1="", "", (A1/1000 + ("1/1/1970"-"1/1/1900"+1)*86400) / 86400)

If you are using UNIX timestamps which only have 1 second accuracy then change

A1/1000 + ...

to

A1 + ...

Adapted from a solution on www.mrexcel.com/forum/showthread.php?t=130691

Thank you thank you thank you! I've been trying forever to make formulas I've found on the net work, and none of them did. You get a gold star. :-)