Daybreak announced it will be fixed:
An update will be required for this change to be pushed out to Live servers. Our next scheduled update is one week from now, on December 12th
Due to the fact that the quest was scheduled to end on December 15, the quest has been extended until December 20.
Unfortunately, the announcement seems to say that Daybreak has shortened the time for some of the other quests in order to end Frostfell at the previously picked date, rather than extending the Frostfell celebration:
We're going to be extending phase one of Frostfell by a few days (until 12/20) and shortening up phase 2 a bit from there.