Please note that the Xron product is now Obsolete. It hasn't being maintained by his author since long time ago. So, I don't guarantee that the Xron product is going to work with zope versions later than 2.6.x. Indeed, somebody told me that there is a problem with the new implementation of the threading module in python 2.3.x. I only let this product here to give you some ideas if sometime you want to do a similar product.
For windows you may use Scheduled task + wget for windows
For Unix/Linux, use Cron + gnu wget
This is just a patch which improves the interface of the Loren Stafford's Xron product version 0.0.10. The schedule mechanism is basically the same with a few changes in the __init__, reschedule, manage_edit, and manage_addXronDTMLMethod methods. I also made a change in the folder structure, I moved the dtml methods to the folder dtml (most of the zope products that I've installed have this folder, so I guess is a standard). The only problem is that the old XronMethods are not compatible with this patch because I added 2 new properties that the old one didn't have: frequency and originalDate, and changed the type of periodDays from float to int. So, you have two options: 1- Before you intall this patch, you can copy your old XronMethods in a text file. After the installation, you can create one by one and paste the code or 2- you can make a script that converts them (I didn't do it, so if you make one, I'd appreciate if you send me a copy). The interface will look like this: