Hello and welcome to my Zilean build.
Before starting, I would like to remind everyone that there isn't a perfect build, regardless of who creates it or how many math there is behind each build. There are only "balanced builds that hopefully will work depending on situations you encounter".
There are alot of variables that can ruin your expectations :
-teammates (unexperienced, stubborn, hot-headed, trigger-happy, one-man-show, and so on)
-enemies (gameplay, coordination, WARDING)
-internet providers (there are no rage-quitters, only players with insufficient bandwidth!! rofl)
-parental units (being asked/forced to tidy your room, eat, make your bed)
-and so on!

I would like to mention that this guide is inspired from Mowen's build for Zilean

I won't copy-paste Zilean's lore just to get a higher character count. You can read that for yourself here!
However, we will be discussing skills and maybe learn (together) some new, or at least fun, facts.

1. Skills:

   • Passive Heightened Learning
     Increases experience gain of all allied champions by 8%.
   • Q Time Bomb
     -effect: Zilean places a time-delayed bomb on a target. After 4 seconds this bomb will detonate, dealing 100 / 155 / 210 / 270 / 330 (+90% of ability power) magic damage to all surrounding enemies.
     -hint: Time Bomb's detonation is not blocked by spell shields such as Sivir's , Nocturne's or a Banshee's Veil, though the initial placement of the debuff is. On the contrary, Morgana's Black Shield does not block the placement of the bombs but absorbs the damage from their detonation.
     
   • W Rewind
     -effect: Reduces all of Zilean's other spell cooldowns by 10 seconds.
   • E Time Warp
     -effect: Zilean slows an enemy champion's movement speed or increases an allied champion's movement speed by 55% for 2.5 / 3.25 / 4 / 4.75 / 5.5 seconds.
   • Ultimate (R) Chrono Shift
     -effect: Marks the target with a protective time rune for 10 seconds. If the target would take lethal damage, they are instead transported back in time regaining 500 / 750 / 1000 (+300% of ability power) of their health.
     -hint: Chrono Shift will always take precedence over other revival abilities, like Guardian Angel and Rebirth.
   
   
   

2. Skill Combo

   • Check Mowen's Zilean build: All About Bombs! (Oh, and those less cool skills) Section
   
   
   

3. Quotes

   • Upon selection
     • "I knew you would do that..."
   • Movement/attacking
     • "All in good time."
     • "Already there."
     • "Inevitably."
     • "Make haste!"
     • "Time is wasting."
     • "Yes?"
   • Taunt
     • "I've seen your death, it was painful..."
   • Joke
     • "Time flies like an arrow; fruit flies like banana."
   
   
   

4. Trivia

   • Zilean's back story is extremely similar to that of The Doctor from Doctor Who.
   • Zilean's ultimate had the most powerful AP scaling in the game at 3:1, it is now 2:1.
   • Zilean's joke is a famous Groucho Marx quote
   • When Time Bomb is placed on an enemy it deals 6 true damage over a period of a few seconds. The order is 3, 2, and 1. This indicates a countdown on the time bomb, similar Sion's Death's Caress.
   
   
   

5. Skins

   • The Shurima Desert skin bears a striking resemblance to Gwonam from the Zelda CD-i games.
   • The Time Machine skin is a reference to H.G. Wells' science-fiction novella, "The Time Machine".

9x Greater Mark of Insight and 3x Greater Quintessence of Potency for a supposed first blood and overall harrass
The 9x and 9x speak for themselves

Overall, this is a decent rune build for most of the supports (even some mages). Thanks to Kazeko, I saw that for myself!

Personal 9/0/21 Masteries tree. Offense tree speaks for itself.
Utility tree without Good Hands and Insight, because I don't plan on dying and I can make the best out of blue in its duration, without needing points in those.
Teleport and Clairvoyance, ergo Spatial Accuracy and Mystical Vision
--I suggest you use Clairvoyance around in the 10th second of the game, on the stairs next to the enemy team's Nexus to see who's laning where, this way you could counter them without losing hp or First blood. Repeated use almost as soon as cooldown's ready (if you didn't ward the map)

I won't go in depth with the item purchase for it may vary, depending on your needs. Situational items will occur. As you may notice I focused on mana regen and balanced armor+magic res, without losing hp, but sacrificing ability power. This build has worked for me, maybe I got lucky, I'm not saying it's the best, I'm not saying it's complete, I just used it and it worked.

There are countless possibilities.

I didn't include wards in the purchase.
Also, don't get Aegis if the tank does, switch to Abyssal Scepter or Frozen Heart.

• Full Ap enemy team?
  ,
  
• Full Ad enemy team?
  ,
  
• Balanced?
  Stick to the core and add situational :
  

• Mine
  • Teleport
  • Clairvoyance
• Other choices
  • Cleanse
  • Clarity
    
  • Exhaust
  • Flash
  • Heal
  • Ghost
    

Matt's -Comprehensive FAQ
Searz's -Runes
Mowen's -Frequently asked question - New player Help
Jebus McAzn's -Who do I buy next?

THE FORUM CONSISTS OF MORE THAN JUST THE OFFTOPIC THREAD!

READ INTO IT!

This is a support guide. Zilean can also be played as a mage, hybrid even AD. Take that into consideration before rushing in to downvote just because it's a different playstyle, with a different perspective.