thanks for good guide :D

Updated the build to follow your advice a bit more. Also fixed an error where the skill order said put a 6th point in time bomb at 13.

Build
The item build and summoner spells are alright, however your skill order is really questionable and I'd prefer a different rune setup, though this is okay-ish. Although as it has been pointed out Zilean doesn't need CD blues and is better off with flat magic resist. Also it's better to go 1-8-21 for masteries, in my opinion, only grabbing Summoner's wrath for the imporves exhaust in offense. Zilean support is pretty much Q W Q into eternity. Maxing Q last makes no sense. You even state 'annoying poke' as a pro, when in reality with 0 AP and having a rank 1 Q until level 10 his poke will be more/less useless. His W and E are purely for utility, and you can max either after Q.
Your build is alright but you should add more depth to your cheat sheet. Like for example, Spawn > Early game > Boot Upgrades > Good Choices > Alternatives > Enchants. Might want to include items such as Twin Shadows, Frozen heart, Abyssal's Scepter, etc. in alternatives.

Guide
Overall your guide is kind of lacking in detail as well as aesthetically. You're putting too much gap between lines of text and you're not adding enough text, as in, not enough content. You've got the basics of coding down, but you should look into columns and spoilers and such. You should also in my opinion go for [[time bomb]] instead of [icon=time bomb] for all of your icons, since this makes people unfamiliar with the icon have to hover over it to see what it is.
Should add more content to general gameplay, or well you only have laning phase so .. :P
You're missing a few key locations in your warding image thing (should center that image, by the way). Could add alternative runes (Armor reds, magic resist blues etc) and in my opinion remove the 'don't be stupid' and 'don't even think about it' etc since they don't add anything to the guide, being obviously bad choices. And in any case Ignite is actually alright on support, while Clarity is not. Riot themselves said clarity is a spell that's only there for new players who can't yet conserve their mana.

It's not an uptvote nor a downvote at the moment. I'll check back on it (you can also message me, if I forget) and if it's imporves I'll drop off an upvote c: Good luck with your guide :3

The guide seems upvoted, though?

Back on topic: I find it astonishing that the author takes 9 points in offense and magic penetration marks, but then doesn't level Time Bomb until last. If you're going to take 9/0/21 masteries (already a bad idea for a no-sustain support), actually take advantage of those 9 points in the offense tree! Time bomb has 320 base damage at rank 5 (640 when double cast), take advantage of it early game or it'll become irrelevant.

Couple minor problems and one major one. Minor ones are runes and masteries. Zilean doesn't need CDR blues. His W resets the timers of Q and E. What he does need, however, is mana regen.

Major problem: you don't specify the order which you build your items.

So that's a -1 from me.