> I also subtract 0:30am from the result?". Subtracting a point in time
> from a duration is as sensible as subtracting weight from litres -
> it's incompatible and cannot be done.

I'm totaly aware of that. I'm using time for duration just because it
simplifies my GUI development. I'll try to improve my GUI and calculate
right field types. It makes more sence.

Anyway, Ivan gave me a very good workarround.

Thanks for both answers.

Regards,
Primoz