Hi,
Your timeDiff formula is returning the time difference for each employee.
So, if you want to see the sum of time difference for all employees for each Company, then you have to insert a group on Company if you haven't created one already.
And insert a Summary on your TimeDiff formula field with sum as option for "Calculate this Summary" dropdown.
Select group footer for group created with Company field under "Summary location" dropdown.
Hope this helps.
Thanks,
Raghavendra