Zilean generates 2 / 3.5 / 5 / 6 / 12 experience (does not count towards level up) every 5 seconds. Time in a Bottle is on cooldown when the game starts.
When the amount stored is enough to level up an allied champion and Zilean has been out of combat for the last 10 seconds, he can right-click the target ally to initiate a 1 second channel (if interrupted, Time in a Bottle goes on a reduced cooldown)
A successful channel levels up the target and grants Zilean the stored amount as experience of his own.
Time in a Bottle marks allies with two circles around their feet; the inner one represents how close they are to leveling up, turning golden when able to activate Time in a Bottle on them.
The target ally is not required to be out of combat to be leveled up by Time in a Bottle - only Zilean is.
Zilean only consumes the amount of experience required to level up an ally. He retains the excess for later use.