AFAIK, I was the one who first mentioned that BSNL meters usage in time blocks, and that if a block starts just before 2:00 am, then usage is counted as chargeable until the block ends and a new one starts.
But I never said that the blocks are all in half-hour units. I've noticed that the blocks can be any length up to 3 hours. It probably depends on the way BSNL adjusts their gateway bandwidth accoding to traffic.
Some blocks are only a few minutes long. This probably happens when the local server's connection fails.
Somehow, as more and more people make posts about BSNL BB, it seems some people make assumptions and post them as facts without verifying them.