RE: Validation on Form

Your approach is incorrect. If I would have to implement this only for the SalesTable form, I would override the init() method on the form, and based on your date criteria range, just disable editing of the form datasources, like locking editing every Monday for example:

if (dayOfWk(systemDateGet()) = 1) // Lock Mondays
{
    SalesTable_ds.allowEdit(false);
    SalesLine_ds.allowEdit(false);
}