We have asked the Administration and unfortunately in this instance, players will have to wait until the next seige to capture the castle.
With regards to the time change, this has already been reported and we are awaiting a fix so that the next Castle Siege (Sunday 02 April) will start as normal at 20:00.
For those who wish to see the timer themselves, this can be done via the Application Room in Wyvern Crossing / Gryphon Crossing.