The simplest time line starts with an arrow and you can add images and text (rotate to allow items to fit in smaller spaces) to your line - each object can be animated to build in and sequenced using the Tools Menu (wrench) > Transitions and Builds option. Tap Done when all the builds are set and Play to see what you've done.
Oops. Sorry, Texgold... The Apple Discussion area used to separate the iPad-specific users of the iWork apps from those on the Mac... since this was posted in what I thought was the dedicated Mac->Keyote forum, I assumed you had a Mac too
As for your question to BW, if I can contribute... you would essentially be creating your timeline by inserting individual lines in the various orientations (horizontal, vertical, etc.). Keynote will offer up guidelines to help you evenly space things and will "snap" objects to equal distances (check your preferences to ensure its enabled), but it's frankly a lot of work and a bit of a pain, depending on how large your timeline is going to be. You can take shortcuts by grouping various items and duplicating them for repetitive patterns.
A faster way may be to scan a ruler (or search the 'net for a public-domain image you can use) and use just the delineated edge, then you'd just have to add the text for days / years / whatever you're planning to do.
A nice place to ask for help and to share with other Keynote aficionados is the "Apple Keynote" Yahoo group (http://tech.groups.yahoo.com/group/applekeynote/)