Seats being released at T-355 over the last few weeks are for the Christmas-New Year period 2023-2024 so yes it is certain they are being taken between 00:00 and 00:10 for popular destinations. There are always J seats released at exactly T-355 at midnight GMT/UTC (so at 01:00 in summer BST). I have been looking at JNB and CPT (very popular) over the last ~2 weeks, there are always 4 J class and 2 W class seats released at midnight on all flights. Once those are taken, it is a dark art as to what seats will be released at any time in the future. Interestingly I tend to see reward flights in J released in big batches for 1-3 months at a time, but usually excluding peak travel days in each direction, e.g. Easter holidays and the school holidays, and sometimes excluding Fri/Sat overnights or Sat/Sun day flights. Based on that, if you can travel late Jan 2024 when demand has dropped you should be able to grab seats at midnight T-355. Otherwise, keep checking every day or so and good luck!