You can record playing time for one or both teams. In the "Game Info" page set the "Track Sub Time" to ON for the team(s) you wish to track. This will restrict the available players to 5 at a time and put a small clock indicator on the control at the top of the collect data page for changing the lineup (the brown icon with 3 players on it). When you enter the screen to change the lineup you can set the current game time and period (touch the digital NEW time to be able to change the period).
You do NOT need to have the game timer running (in the collect data page) to track playing time. You only need to set the correct period and time when changing the lineup as noted above. The clock in the substitution page will default to the current timer setting however which you may find useful.