I'm a bit more certain about the midnight, because of the way it was described.
The only reason I hesitated over the 15 min bin is because my own router tends to show it from uptime, rather than to the current clock time... but then again the DSLAM will be doing its own recording and gathering of stats for DLM.
This is why we err on the side of caution and say 30mins, just to ensure that the DLM sees at least one empty 15min bin to cover any discrepancies between the two methods.