A bar has always an open, hi, low, and close price. If you have 1-hour bars that start at every full hour, there is no 12:45 bar. There is only a 12:00 and a 13:00 bar. Otherwise you had no bars, but just a continuous price curve.

By convention, the time associated to a bar is its close time. This is the time when the "run" function is executed. Thus, a 12:00 bar is not the bar that begins at 12:00, it's the bar that closes at 12:00.