It is very likely that your modem provides statistics about amount of data downloaded and uploaded. This information can be reset to zero by issuing a command while logged in to the modem or every time the modem is restarted. So, if you want to know your downloads daily, log in to your modem daily (at the same time) and have a look and then either reboot the modem or zero the count or note down the values and calculate by subtracting current values from previous day values.
