Where inside my page layout i need to register Page.ClientScript.RegisterOnSubmitStatement?

Question

I were searching for an approach to add a validation on a Date field inside my enterprisewiki.aspx page layout, the validation to check if the Date >= Today. so finally i find this appraoch http://frontendfreaks.com/custom-validation-in-sharepoint-2013-page-layouts/ .. but it is not clear to me how i need to apply this inside the page layout? mainly where inside my page layout i need to add this code:-

Page.ClientScript.RegisterOnSubmitStatement(this.GetType(), "validate_page", " if (!validate()) { return false; }; ");