# 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

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.

> **ERROR**: XRechnung 3.0

## Context and Significance

This EN16931 core rule is the line-level counterpart of BR-CO-19, preventing an empty line period group with no delivery or service timeframe.

## Solution

When a cac:InvoicePeriod appears inside an invoice line, provide at least a StartDate (BT-134) or an EndDate (BT-135).

- validate
- convert

## 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>
    <cbc:DescriptionCode>3</cbc:DescriptionCode>
  </cac:InvoicePeriod>
</cac:InvoiceLine>
```

## Affected Fields

- [BG-26 Invoice line period](/resources/xrechnung)
- [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://erechnungsvalidator.service-bw.de/): Validate your XRechnung files against all BR-DE rules.
- [Convert PDF to XRechnung](/pdf-to-xrechnung): Automatic validation error fixes when converting.

## Related Resources

- [Official Specification](https://xeinkauf.de/xrechnung/)
- [Test Suite](https://github.com/itplr-kosit/xrechnung-testsuite)
- [Complete Error List](/resources/kosit-validator-errors): Overview of all German XRechnung validation rules.