After siege ending a clan leader who castle owner's in 24h period can setup the siege date from siege's date list. This is specified in Siege rules. But server don't processing siege data from client. So, you can craft the packet from server CastleSiegeInfo to client (if you want select the crafted date in ComboBox element) or from client RequestSetCastleSiegeTime to the server and setup your custom date. Checked on L2OFF Vanganth IL pack. Period for changing siege date not processing aswell and you can make a changes in anytime and unlimited times. All what changed after 24h, it's only hidding the ComboBox for selection siege date in client, but... Was crafted date in "Sunday, 21:08" and many others.
Possible this works on the other L2OFF packs.
Have fun!