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 + ...