Have you already tried power cycling the modem? That might be all you need to do to force it to update the time, so if you haven't done so already, please give that a try. To do that you can just unplug the power for about 30 seconds or so to reset it. If that doesn't fix the issue, please let us know. Thanks!
There are time settings somewhere in the menus.. I haven't looked at my settings for a long while and going off memory..
There are NTP/SNTP settings in the time menus. Make sure that NTP is enabled, if you need to enter a server, try pool.ntp.org.
Daylight savings still 'active' in December seems odd.. So that is what I would be checking first, that it actually has the right time, the correct time-zone and such.