The time used by ForeFlight in Track Logs is Epoch Time, which is part of a software timekeeping system. The Excel formula to convert the epoch time to actual date/time is:
If you enter a column for time correction into your spreadsheet, be sure to set the cells to a date/time format.
Imported CSV file showing an uncorrected Epoch timestamp.
CSV File with column added showing Epoch time correction.
The time displayed will be in GMT time. If you want to convert to a specific time zone, you can adjust the formula used to look as follows:
=CELL/(60*60*24)+DATE(1970,1,1)-TIME(HOURS TO ADJUST,0,0)
Here is a sample of an actual formula used with a time subtraction of 5 hours: