StarBunnie's post in Is there a plugin to make a region in the same world specifically night? was marked as the answer
Worldguard has such a feature with the flag time-lock.
To use it just type:
/rg flag <regioname> time-lock <time in ticks>
Time of day in ticks (between 0 and 24000) that players will see the world as while in the region. Use + or - for time relative to the world time. [Link to worldguard flag wiki]