Running the most recent version on my WNR3500L.
Settings / Bandwidth (doesn't matter whether it is daily,monthly, weekly) - if I lose power, then the accumulated use for this month resets to 0. Apparently there is no cumulative logging and rollover on a monthly basis.
When I ran DDWRT (on my old Cisco router), it retained monthly use even if the router were powered down. And I could consult previous months usage if I wanted.
Is this a setting on Shibby/Tomato? Can bandwidth data be retained?
Can't help you with a solution, but can tell you why it happens. :)
The saving of history defaults to "RAM (Temporary)". Check Administration/Bandwidth Monitoring" and Administration/IP Traffic Monitoring in the UI.
I too would like to know how to configure these settings to save/restore the history automatically to/from something like a USB Flash drive plugged into the router or the router's own flash memory.