monitor network traffic

Hi,

Here in Belgium internet poviders have bandwith limits on all the accounts. If a person go over the limit, the provider puts you on smallband
My account has a total of 30 GB/month
I would like to have a program that monitors my network traffic, so I have total control of my account.
There are programs like 'DU meter'. This program is a real help, but there's one problem.
Between 00:00 and 10:00u at night, my provider allows us to download more: every 2mb counts as 1mb.
I didn't find any program where you can set the time for monitoring, so I want to make one myself.