The only way I know that a citadel time changes like that, is when it resets later if no one enters at the time of the original tick.
In your case, assuming no one else entered until the time of your event, the tick would be pushed forward a half hour to 8pm. So this doesn't explain the movement of the tick to almost midnight, unless no one entered until then, or that its crept forward each week where you have moved the event to half an hour after the tick.
There is a very long process to pull the time forward - and that involves actually being in the citadel at tick out time for the tick, then immediately re-entering. This should pull the tick forward by 3 mins or so a week - so if you are getting resources easily, it probably will be better to reset the tick to your original time, and pay the cost.
To avoid further movement, make sure someone enters as soon as possible after the citadel has ticked
Kera:
Proud leader of Shattered Moon