Excel dates work on a similar principal, but they measure day in terms
of a reference date of 31st December 1899 (day 0), so you will have to
add the number of days that your reference date represents in Excel.
Enter 1/1/70 into a cell and format it as a number - this will show you
that it represents 25569 days, so this is what you will have to add to
your elapsed days to make it into Excel dates.

Times are represented as fractions of a 24 hour day, so to convert your
seconds you will have to divide by 60, by 60 again and by 24. So, if
your timestamp is in cell A1, this formula in B1 should do the trick:

=A1/60/60/24 + 25569

Format the cell as Date in the appropriate style.

Hope this helps.

Pete