you can do that to save some usage while browsing in the mornig, but don't rely on that for heavy downloads. Generally if I reconnect at 7:59 it is not charged upto 8:29, but methods can change anytime. And anyways, these people have strange billing methods, you cannot rely on what is shown in the portal. So i would suggest that if you do that, only do some surfing, don't download in that period. just to be on the safer side.