Custom new form validation on Save Ok button

Another undocumented piece of SharePoint.

I want to validate two fields on a new list item form by invoking JavaScript custom function. They are two date fields and I want to ensure that the end date can't happen before the start date. My first idea was to attach a validation function on the onclick event of the Submit button.

I started by inspecting the generated HTML of the form. The Submit button already has a onclick() code which is:

if (!PreSaveItem()_) return false;WebForm_DoPostBackWithOptions(new WebForm_PostBackOptions("ctl00$ctl13$g_740c1963_b0da_4b45_9b71_0dcca5d082b0$ctl00$toolBarTbl$RightRptControls$ctl00$ctl00$diidIOSaveItem", "", true, "", "", false, true))

Searching in the SharePoint JavaScript files in the LAYOUT folder, I found the definition of PreSaveItem function in FORMS.JS file. It simply invokes PreSaveAction function, if defined.

Finally, it was just a matter of inserting a custom function named PreSaveAction in a