Devrath is right. IOW, BSNL bills you for a period up to midnight on the last day of the month. It makes no difference when you first started using your connection in the first month.
There's one thing I've discovered about BSNL's billing: If you go online before midnight of the last day of the month and continue without break after midnight, your usage between midnight and 2:00 am may not be included in the bill for the last month or the next. But it will be included in a later bill.
As an example, suppose you go online at 10 pm on the 30th June and continue up to the morning of the 1st July. Your usage between midnight and 2:00 am of the 1st July will probably not be included in your bill for June or July. But it will probably be included in your bill for August.
Of course, it makes no difference if you don't exceed 2.5GB in either June or July. But if you do exceed it, you may get an unexpected increase in your bill for a later month.
