@The_DooM: That's not really the best method to do this.
It would be better to have an infinite duration then a repeated check throughout the night to see if units have the buff.
Once it's daytime, do a group enumeration that removes the buff.
By the way, never use Do Nothing.
The easiest method to apply the speed bonus is to use Endurance Aura and make the targets affected Self only. However, that brings the trouble of using auras for buffs since the levels aren't automatically set, and too many auras cause lag.
@Blackrockorc: Could you be a bit more specific?
Like, is it only one specific unit or several units? The trigger would be more simple if it only worked on one unit.