How to interpret timestamps stored as 10-digit negative integers?
Per Your comment the only place the values show as time stamps is in the application. You don't have access to the applications code.

The only possible solution, is to get a handful of actual time values shown in the application and relate them to values in the field, than figure how the difference is calculated. Then you will need to recalculate the values and add them to your database.  

I strongly suggest that you do not replace the source data.  Possibly add columns to the table with the correctly formatted dates.