I am using the freeware, networx to track usage data.
1>Go to cnet.com, then the downloads section
2>Search for networx; download and install it
3>Should start-up automatically, otherwise put
it in the startup folder, so that it's always
running
4>In the program, click on Settings->Advanced.
5>Configure the on-peak(hours) as between 6-21 and
the beginning of months as your billing start-date.
6>Configuration is finished. Whenever you are
logged on, click on UsageStastics then
on Daily/Weekly/Monthly Report "Display Options"
it shows On-Peak/Off-Peak usage data.
Disclaimer:This program works well for me.
BUT, I am not affiliated in any way with the
software/company, so any errors/bugs/viruses are NOT
my concern/responsibility.