Seems more complex than that.
Yesterday afternon (4th Aug) I looked at what I had used on 'myBT'. 2.56 GB download, 3.74GB upload. The page also said "Last usage update:03.08.2015 at 23:59".
This morning I looked again and it is now over 6GB download and 5 GB upload. The page still says "Last usage update:03.08.2015 at 23:59" !!!
No, it hasn't, not this month anyway. I wasn't pointing out (in this thread at least) about the amount of data, but that the report of the amount of data is changing when the timestamp is still the same, which makes a nonsense of the timestamp.
My concern about the amount of data itself is covered here and I'm still waiting for an answer