your data allowance is divided into 2 periods:
Anytime Bytes: 8am to 2am local
Bonus Bytes: 2am to 8am
You really have THREE data sources to draw from if you include any amounts from extra cost "Token Bytes"
(click on picture for larger image)
You need to monitor your usage rate so as to pace yourself.
To that end you have three usage meters:
One is internal to the modem and can be accessed by 192.168.0.1
This is known as the SCC (System Control Center)
The second meter is a utility you can download and install. This is known as the Download Status Meter and can be found on your "dashboard":
Once installed it looks like this:
The third and most accurate is also found after logging into your dashboard and looks like this:
On a data capped service you simply have to monitor and control your usage.