I want to create a trigger in foxpro that will execute after every update in my db table, for example JobActivity.After every update, the trigger should insert the update time in the table JobActivity.
Instead of AFTER the update, you could apply a Record RULE validation (as @Stuart Dunkeld mentions) that is applied against the entire record before the insert/update is processed.
Go to your database container and create a stored procedure something like
FUNCTION SP_LastUpdated()
replace LastEditDT WITH DATETIME()
RETURN .t.
ENDFUNC
Then, for the record validation rule, put in SP_LastUpdated().