On December 11, according to the WDW calendar (click
HERE to see the calendar -- or simply click "calendar" on the blue bar toward the top of the Moms Panel web site), the Magic Kingdom will be open for normal operating hours from 9 am to 7 pm. The calendar also mentions that there will be Extra Magic Hours (EMH) on that date. By clicking the link for Extra Magic Hours, I find that the Magic Kingdom has
Morning (not evening) Extra Magic Hours at the Magic Kingdom on December 11. The morning EMH always start one hour before the normal operating hours (in your case, the park will open for EMH at 8 am).
The evening EMH hours are "up to three hours" after normal park closing hours. In my experience, it is always EXACTLY three hours extra -- I have never heard of it being less.
Simple rule: the WDW calendar shows the normal operating hours. EMH will be before or after those hours.
Hope that helps. ºoº