A few suggestions

You're right about TIMER. Number of seconds since last midnight. Accurate to approximately 1/18.2 of a second.

If you want more accurate timing you can read the PIT counter that gets updated about a million times a second, but this is tricky. I can dig up some details if you want.

There is also the screen refresh interval which is usually 60Hz. I don't think this method is going to suit your application.

QB64 allows more precise timing with _TIMER and other things. I would suggest you look into this unless you are using DOS.