so u can see that log on time after 2am (which is to be taken as unlimited download) is taken in as chargeable usage whereas it shud be only from 1:52 to 2 am that shud be chargeable and from 2 am onwards it shud be under unlimited usage
See as you already have noticed user sessions rounded to 30 minutes. So if you connect at 1:59AM and continue with the same session, then for the next 30minutes, ie, upto 2:29AM it will be considered as PAID session, as you ve logged-in in a PAID time period.
Similarly if you connect at 7:59AM and will continue with that session then for next 30minutes,ie, upto 8:28AM it will be considered as FREE usage as you have logged-in in a FREE time period.
Hope its clear now. So its better to have a session like .....
2:02AM (connect) -> 7:57AM (disconnect) &
7:58AM (connect) -> 8:26AM (disconnect)
:dance::yahoo::dance:
NOte: Keeping 2 minutes in ourside in each session, for possible mismatch of time sync with BSNL server