How to configure Internet usage meter?

[Lomax]

My router reports bytes sent/received as an accumulated integer, and I would like to track the daily usage in Domoticz. I added a virtual "RFXMeter Counter" but every time the router reports usage this is added to the previous total, so yesterday I used (apparently) 18446744073709552000 bytes download... To further complicate things, this is a WWAN router and the byte meter will be reset every month, which I suspect would result in a negative traffic figure for the following day?