I'm sorry, because I was speaking nonsense before. Dividing with dbScreenFPS() is unreliable. You should of course subtract the new timer by the old timer.
Note that the order these commands are called is very important for the functionality to actually work.
In the small test app I had, with some minor 3D operations, the delta time was 17 milliseconds. I.e., 0.017 seconds, which would be the multiplier used every frame, as g_Delta.
static int g_OldTimer;
static float g_Delta;
while ( LoopGDK ( ) )
{
g_Delta = ( dbTimer() - g_OldTimer ) / 1000.0f;
dbSetCursor( 0, 0 );
dbPrint( g_Delta );
g_OldTimer = dbTimer();
dbSync ( );
}