One way would be to use the GetTickCount api call. GetTickCount returns the number of milliseconds since the machine was booted.

Code:
Public Declare Function GetTickCount Lib "kernel32" Alias "GetTickCount" () As Long
When your app first loads make the call. Then at any point in the future, make the call again. Second - First equals you app time in milliseconds.

Code:
Dim lRtrn1&, lRtrn2&

  lRtrn1=GetTickCount
  '/
  '/...
  '/
  lRtrn2=GetTickCount

  msgbox lRtrn2-lRtrn1 & " milliseconds between calls!"

td.