what particular documentation is the cron manager tutorial can be found?
or is it possible that i can set a time in the cron manager to raplce a certain banner in the exact given date and time? (ex. august 1, 2010 1pm in sydney)
See here for information relating to Future Status in a couple of the manuals:
http://manuals.matrix.squiz.net/asset-screens/chapters/details
http://manuals.matrix.squiz.net/triggers/chapters/example-setting-a-future-status-of-up-for-review-for-a-user-account
The way you probably want to do this is create a new image/file asset (the new banner), edit the asset that's referring to it using Safe Edit, save changes (and approve, if in a workflow) and then set a Future Status to "Live" for the time you want it to go live.
Don't forget to factor in caching if you have a long cache period and may need to set it live a bit early (or schedule a clear cache cron job as well):
http://manuals.matrix.squiz.net/tools/chapters/tools
http://manuals.matrix.squiz.net/tools/chapters/clear-matrix-cache