How to set a mouvement at a specific moment of an animation

For example, selection X has to move from position A at t=10s to position B at t=15s. Even if selection X did not moved from t=0s to t=10s, a position key must be setted at t=10s. So seemage will knows from when to start the movement. Otherwise, it will start from the previous position key for this selection X which, in that case, is t=0s.