convert serial number to hr min sec in excel 2003

Hi, I am getting a list of numbers in a data feed that i believe may be serial numbers for time values. e.g. I think that:

1323648006 = 12:00:06 AM
1323651792 = 1:03:12 AM

Can anyone help me with the calculation to convert them into the hh:mm:ss format that i need?

Thank you