# BR-CO-20 Invoice line period needs a start or end date

If the invoice line period (BG-26) is used, the invoice line period start date (BT-134) or the invoice line period end date (BT-135) shall be filled, or both.

## Problem Description

> **ERROR**: EN 16931 calculation and consistency rule (BR-CO)

## Context and Significance

This EN 16931 rule is the line-level counterpart of BR-CO-19: an invoice line period (BG-26) must not be empty. Unlike the document level in UBL, a line period has no exception for cbc:DescriptionCode, because the VAT point date code (BT-8) exists only at document level. The error often occurs when an export creates a period element for every line, even for lines without a known period.

## Solution

When a cac:InvoicePeriod appears inside an invoice line, provide at least a StartDate (BT-134) or an EndDate (BT-135); otherwise remove the empty element. In CII the line period is ram:SpecifiedLineTradeSettlement/ram:BillingSpecifiedPeriod.

- [Validate XML](/en/xrechnung-validator?source=resource&rule=BR-CO-20)
- [Convert invoice](/en/pdf-to-xrechnung?source=resource&rule=BR-CO-20)

## Valid Examples

- BT-134 = 2026-02-01 with BT-135 = 2026-02-28

## Invalid Examples

- Line cac:InvoicePeriod present but neither BT-134 nor BT-135 filled

## Code Examples

### Valid XML
```
<cac:InvoiceLine>
  <cac:InvoicePeriod>
    <cbc:StartDate>2026-02-01</cbc:StartDate>
    <cbc:EndDate>2026-02-28</cbc:EndDate>
  </cac:InvoicePeriod>
</cac:InvoiceLine>
```

### Invalid XML
```
<cac:InvoiceLine>
  <cac:InvoicePeriod>
  </cac:InvoicePeriod>
</cac:InvoiceLine>
```

## Affected Fields

- [BG-26 Invoice line period](/resources/xrechnung/bt-134-invoice-line-period-start-date)
- [BT-134 Invoice line period start date](/resources/xrechnung/bt-134-invoice-line-period-start-date)
- [BT-135 Invoice line period end date](/resources/xrechnung/bt-135-invoice-line-period-end-date)

## Related Rules

- [BR-CO-19](/resources/kosit-validator-errors/br-co-19-invoicing-period-date-required)

## Validation Tools

- [Official KoSIT Validator](https://github.com/itplr-kosit/validator): Run the official KoSIT validator with the XRechnung configuration to check EN 16931 and XRechnung rules.
- [Convert PDF to XRechnung](/pdf-to-xrechnung): Find validation errors during conversion and review each correction.

## Related Resources

- [Official Specification](https://docs.peppol.eu/poacc/billing/3.0/rules/ubl-tc434/)
- [Test Suite](https://github.com/itplr-kosit/xrechnung-testsuite)
- [All validation errors](/resources/kosit-validator-errors): Overview of the EN 16931, XRechnung and Peppol validation rules explained here.