I think I'm correct in saying that UNIX time is the number of seconds that has elapsed since 1/1/1970. Excel calculates dates by using the number of days that has elapsed since 1/1/1900. Therefore you should be able to convert from one to the other by converting from seconds to days, and then adding on the 70 odd years difference.
The formula :- =(A1/86400)+25569 where A1 contains the UNIX time should convert to Excel date/time. Make sure you format the cell as the required date/time format.