I mean, hey, it's fun zooming around and spam-laughing at the enemy when you're two feet in front of them with 700+ movement speed, or slowing them down for 5 to a basically 110 movement speed, which is even more annoying than being rooted for that long.
We all know and love Morgana's bind, right?

This build is meant to take off some pressure of your shoulders, or just to warm you or your teammate up for the games up ahead.
People like taking Zilean support as an APC, but in my opinion, isn't it better to mentally torture your opponents with your constant slows, stuns and revives? Let me remind you that Zilean has a cooldown of 33 seconds on his ult with 45% on level 16, and stopping someone important dying in that phase of the game is really vital to winning.

Summon Aery is used for shielding and poking enemy early in the game.
Manaflow Band Zilean struggles with mana when you don't have mana regeneration early in the laning phase, so you're going to poke with:
Time Bomb if the enemy laners are always behind their minions, or
Time Warp if they're in the middle or pushing, since you can easily engage with Time Bomb stuns afterwards.
Transcendence is perfect for the early additional 10% Cooldown Reduction, since you're not trying to really engage before you get your "power spike" of spamming abilities.
Gathering Storm is focused for lategame, and that's what you're aiming for.
Magical Footwear is a good boost to Zilean's base speed of 335 movement speed. You gotta have proper boots to resist the slowdown abilities as well, these built into Boots of Swiftness works perfectly, and you don't have to worry about using 300 gold to buy them, you can focus on getting Control Wards and a Refillable Potion.
Cosmic Insight is what makes you even more annoying later in the game, giving you Cooldown Reduction on every single thing in your kit and raising the cap by 5%, it's nothing special early, though.
Ability Haste Cooldown Reduction, obviously.
Adaptive Force Adaptive Force for a bit of a harder poke or a better shield, but it ain't much, you can switch it for some Shield since you won't be building it until lategame, or at all.
Flat Health Health is not that bad since he's squishy, but if you're going against a full AD team, you can go for both Shield