This will not work as the timer tick events will not fire while other code is running (unless you add the DoEvents keyword).
Just checking the value of Now will also not be enough because it is updated each second on the device.
The solution is to use GetTickCount from this library:
http://www.basic4ppc.com/forum/additional-libraries/384-useful-library.html
Calculate its value in the beginning and end of each sub.