# BR-29 Invoicing period end must not precede its start

If both Invoicing period start date (BT-73) and Invoicing period end date (BT-74) are given, BT-74 shall be later than or equal to BT-73.

## Problem Description

If both Invoicing period start date (BT-73) and Invoicing period end date (BT-74) are given, BT-74 shall be later than or equal to BT-73.

> **ERROR**: XRechnung 3.0

## Context and Significance

This rule comes from the EN16931 core semantic model enforced by the KoSIT XRechnung 3.0.2 Schematron, ensuring invoices carry the mandatory data needed for automated processing.

## Solution

Correct the dates in cac:InvoicePeriod so cbc:EndDate is on or after cbc:StartDate, or remove the period if it was entered in error.

- validate
- convert

## Valid Examples

- BT-73 = 2026-01-01, BT-74 = 2026-01-31

## Invalid Examples

- BT-73 = 2026-01-31, BT-74 = 2026-01-01 — end before start

## Code Examples

### Valid XML
```
<cac:InvoicePeriod>
  <cbc:StartDate>2026-01-01</cbc:StartDate>
  <cbc:EndDate>2026-01-31</cbc:EndDate>
</cac:InvoicePeriod>
```

### Invalid XML
```
<cac:InvoicePeriod>
  <cbc:StartDate>2026-01-31</cbc:StartDate>
  <cbc:EndDate>2026-01-01</cbc:EndDate>
</cac:InvoicePeriod>
```

## Affected Fields

- [BT-73 Invoicing period start date](/resources/xrechnung/bt-73-invoicing-period-start-date)
- [BT-74 Invoicing period end date](/resources/xrechnung/bt-74-invoicing-period-end-date)

## Related Rules

- [BR-30](/resources/kosit-validator-errors/br-30-line-period-end-after-start)

## 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.