# UBL-SR-36

> Invoice line period shall occur maximum once

Checked against Peppol BIS Billing 3.0 2026.5

Canonical: https://einvoicecheck.eu/rules/ubl-sr-36

## Official rule text

Invoice line period shall occur maximum once

## Why did my invoice fail UBL-SR-36?

An invoice line may carry an Invoice line period (BG-26), the billing/delivery period for that line, expressed with a start date (BT-134) and end date (BT-135). EN 16931 allows at most one period per line, so cac:InvoicePeriod must not occur more than once inside a single cac:InvoiceLine.

## Common causes

- Two cac:InvoicePeriod elements on the same invoice line.
- A recurring-service template emits one period per month on a single line instead of separate lines.
- Both a delivery period and an invoicing period written as two InvoicePeriod blocks.

## How do I fix it?

```
Keep one period per line: <cac:InvoicePeriod><cbc:StartDate>2026-06-01</cbc:StartDate><cbc:EndDate>2026-06-30</cbc:EndDate></cac:InvoicePeriod>; use separate lines for distinct periods.
```