Bob,

You can implement this by adding an odoo constraint to check if the start date entered is of previous month then , raise an error...

your constraint: _constraints = [(_check_date_validation, 'YOUR_MESSAGE', ['date_from', 'date_to'])]

method:
def _check_date_validation(self, cr, uid, ids, context=None):

    for self_obj in self.browse(cr, uid, ids, context=context):

        date_from = datetime.strptime(self_obj.date_from, '%d/%m/%Y').date()

        # datetime.now().month is your current month and

        # date_from.month is the month selected in the from date

        if datetime.now().month > date_from.month and date_from.day==1:

            return False (or you can raise exception here only to display custom message)

        elif YOUR OTHER CONDITIONS

            return False (or you can raise exception here only to display custom message)
    return True

Hope it helps!