That guide aims to build a Zilean that can support and, if needed, to resist a little but at the same time that can make the damage that a real zilean should make. That is in my opinion the way to build a successful zilean but is just an opinion so please try and analyze this build before rating, a comment would be appreciated.

PROS

Great harasser;
Great support;
Good resistance;
You have a lot of ways to run away;
He has got one of the most annoying ultimate in the game;
Not so difficult to play;
Great farmer.

CONS

He needs a good timing for support and harass well;
Squishy early game(but not that much);
Spamming a lot as you should do will take you a lot a problem with mana;
Not that effective as other supports such as sona or soraka.

I'm in love with Flash and Teleport: the first one let you to escape from bad situation or combined to Time Warp to reach easly a teammate in danger; Teleport instead let you to don't waste time going back to the base to buy items or heal while laning and let you to reach fast your teammates.

Other good choises

Clarity could be a good choise but it will be useless in the late game
Clairvoyance is so good to support, map control is essential
Ghost works very well with your Time Warp

Nothing special to say about it, tipical mages runes.
I use 3 Greater Seal of Replenishment and 6 Greater Seal of Scaling Mana Regeneration because they will help you in early game without damage your mid/late game mana pool.
You can also replace Greater Glyph of Cooldown Reduction with Greater Glyph of Scaling Ability Power that aren't so bad.

Typical mages masteries: more mana regen, more CDR, more EXP, more speed... nothing new...
Just remember to change them depending on your summoner spells.

Early Game

Begin with Doran's Ring that is a really good item for the early game: it has all you need, mana regen, healt, AP... it is great to begin to harass.
Lane as much you can if you resist well in lane at your first back you should be able to buy directly full shoes although your teleport let you to go back and forth from your lane really fast. Anyways now you can buy several diffent shoes, in my opionion Mercury's Treads are the best 'cause of tenecity that is very helpful, but if you prefer my suggested ones are Ionian Boots of Lucidity and Boots of Swiftness, the first one synergize well with your rewind and Boots of Swiftness are very helpful on a support(obviously i don't suggest them if you already got teleport to reach your teammates faster). As regard Sorcerer's Shoes i don't really suggest them cause you are a support you don't really need other magic penetration than your runes give you.
The second time you go back you should be able to buy Catalyst the Protector that is very helpful too in the early game.

In early game this should be your core build:
Catalyst the Protector

Just remember: if your team or you are going good you should buy Mejai's Soulstealer that is ever a good choise.

Mid Game

Now you should rush for Rod of Ages: this is a great item on zilean more HP, more Mana, more AP, not so bad, isn't it?
Even now you should be able to support very well but it's time to buy the real support one item: Aegis of the Legion. Guys that item is so good for your team and for you too, giving a lot of resistence both, especially in that phase of the game.
Then buy a Negatron Cloak that garant you a little more resist against magic sourches.

You should look like this now:

Late Game

Ok guys time to complete your Abyssal Mask, another great aura item for Zil.
After that buy Zhonya's Hourglass that has a really amazing active. Completing that items build you'll be really tanking becoming a very competitive support offtank.

The sixth item:

Time to sell Doran's Ring. If you got Mejai's Soulstealer you won't have the problem of choosing. If not you should buy it depending on the situation: want more AP? Buy Rabadon's Deathcap. Want more resistance? Buy Guardian Angel.


Other Viable Items:

You can consider Archangel's Staff if you aim to have more AP, it gives you a lot of them and an ever useful mana regen.

If you got other shoes than Mercury's Treads you should get a Moonflair Spellblade to compensate the lost tenacity.

Another viable item could be Morello's Evil Tome ever beacause of the CDR and mana regen.

If you got so much magic champions in the team a Will of the Ancients could be a good choise to support them aswell.

If you really can't stand against the enemy team AD champions you just should get a Thornmail(just if you are the first to be focussed).

Banshee Veil and Quicksilver Sash are good items too but you should really look at the situation.

Time Bomb is your most important skill, after your Chronoshift obviously. This skill is very important for harass and farm and is your only sourch of damage... but this skill would be nothing without your Rewind wich let you use your Time Bomb twice in a row dealing an insane ammount of damage...


Chrono Shift: When should i use that?

As for all the skills it depends on the situation u will usually use this to help a dieing teammate, but you can use it too on a full life teammate that could be shootted as soon as got in the fight... you just need to catch the right moment and have a good timing... The same applies to your Time Warp. Remember the right timing on your skills can change a team fight.