period_from = models.DateField(verbose_name='From')
period_to   = models.DateField(verbose_name='To')

class AdminBillForm(ModelForm):
    class Meta:
        model = Bill
        exclude = ()

    def clean_period_from(self):
        period_from = self.cleaned_data.get('period_from')
        if (period_from >= timezone.now().date()):
            raise ValidationError("'From' date cannot be a future date.")
        return period_from

    def clean_period_to(self):
        period_to = self.cleaned_data.get('period_to')
        if (period_to > timezone.now().date()):
            raise ValidationError("'To' date cannot be a future date.")
        return period_to

    def clean(self):
        cleaned_data = super(AdminBillForm, self).clean()

        period_from = cleaned_data.get('period_from')
        period_to = cleaned_data.get('period_to')

        if (period_from >= period_to):
            raise ValidationError("'From' date should not be beyond 'To' date.")

@admin.register(Bill)
class BillAdmin(admin.ModelAdmin):
    form = AdminBillForm

'>=' not supported between instances of 'NoneType' and 'datetime.date'