The following events influence happiness:
-Winning a mission with enough surviving civilians slightly increases happiness of all nations, strongly for the nation where the mission was.
-Winning a mission with not enough surviving civilians (or even losing it) slightly decreases happiness of all nations, strongly for the nation where the mission was.
-Selling an UFO slightly increases happiness of all nations, strongly for the nation it was sold to.
-An active mission, which is not a crashed UFO mission, strongly decreases the happiness of the nation where it is located at every midnight, until it is flown. There also seem to be some missions which, in addition to the former, very strongly decrease happiness in the moment they disappear or the UFO takes off.
As long as nothing of these happen, happiness doesn't change. Just that time is running means nothing.
It's also possible to win missions with just the right number of surviving civilians so that the happiness impact is zero. Missions without civilians also always have zero happiness impact.