# BT-73 Invoicing period start date

Start date of the period to which the invoice applies.

## Quick answer

Start date of the period to which the invoice applies.

> **Delivery and payment**: Delivery dates, payment means, and the operational context around settlement.

## In plain language

Start date of the period to which the invoice applies.

Start date of the period to which the invoice applies.

Important for subscriptions, rentals, and all invoices that bill a service period instead of a single delivery moment.

## What you usually enter here

ISO 8601 date format (YYYY-MM-DD)

Typical valid values include: 2026-04-01, 2026-01-01

Optional according to EN 16931-1.

## What goes wrong often

Avoid values like: 01.04.2026, 2026/04/01

## Examples in context

Important for subscriptions, rentals, and all invoices that bill a service period instead of a single delivery moment.

Optional according to EN 16931-1.

## Valid Examples

- 2026-04-01
- 2026-01-01

## Invalid Examples

- 01.04.2026
- 2026/04/01

## Related Fields

Important for subscriptions, rentals, and all invoices that bill a service period instead of a single delivery moment.

- [BT-74 Invoicing period end date](/resources/xrechnung/bt-74-invoicing-period-end-date)
- [BT-72 Actual delivery date](/resources/xrechnung/bt-72-actual-delivery-date)

## XML mapping and field facts

Open the technical reference if you need XML paths, validator rule codes, legal wording, or schema-specific mapping details.

### Field ID:
BT-73

### Data Type:
date

### Cardinality:
0..1

### Requirement:
Optional

### Since Version:
Optional

## XML Implementation

### UBL 2.1 Syntax
```
/Invoice/cac:InvoicePeriod/cbc:StartDate
```

### CII (UN/CEFACT) Syntax
```
/CrossIndustryInvoice/SupplyChainTradeTransaction/ApplicableHeaderTradeDelivery/ram:BillingSpecifiedPeriod/ram:StartDateTime
```

## Why the standard cares about this field

### Business Context
Important for subscriptions, rentals, and all invoices that bill a service period instead of a single delivery moment.

### Legal Requirement
Optional according to EN 16931-1.

## Official sources and tools

- [KoSIT Validator](https://erechnungsvalidator.service-bw.de/)
- [Invoice Converter](/pdf-to-xrechnung)
- [XRechnung Standard](https://xeinkauf.de/xrechnung/)
- [Test Suite](https://github.com/itplr-kosit/xrechnung-testsuite)