Bandwidth monitoring utilities - Questions

[pftdm007]

Hello,

I recently discovered that it is possible to monitor my monthly bandwidth consumption of my WAN connection.  ON this forum, I've found vnstat which does pretty much what I need, except that like some other users, my monthly billing cycle does not start on the first of each month but on a different day (the 4th)…  So my question in regard to vnstat is:  can you change the start date ?

Also, I tried the RRD summary and seems to be decent feature but the numbers appears WAYYYY off.  For example, I changed the start day to the 4th and it says that I downloaded 49.5GB since the 4th, yesterday, which is totally impossible since all my machines were off during that time, and my ISP reports 0GB (probably its too low to be reported)...  So what could be the problem with RRD summary?

Thanks for any advice on this!