If it is a short term timer, like one I made, you might use the KeepAlive function. You need to be sure to release it when your app is done. Basically, it stops the screen from turning off until your app is over. I like this because I need to keep an eye on my timer. Here is the code that I gleaned from the forum:
Sub Process_Globals
'These global variables will be declared once when the application starts.
'These variables can be accessed from all modules.
Dim Awake As PhoneWakeState
Sub Activity_Create(FirstTime As Boolean)
Awake.KeepAlive(True)
Sub Activity_Pause (UserClosed As Boolean)
Awake.ReleaseKeepAlive
End Sub
Jim T