The cut-off seems to be based on the time-zone of the departure airport.
For the UAE, it seems that they stop sales at 8pm local time for departures on the same day. This means that on 8pm on a Monday, you can buy the 0030 departure from DXB on Tuesday, but not the 2335 departure on Monday out of AUH.
Other non-QR websites are probably able to sell you the tickets, though.