# BT-73 Invoicing period start date

Start date of the period to which the invoice applies.

## Quick answer

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

- [Validate XML](/en/xrechnung-validator?source=resource&bt=BT-73)
- [Convert invoice](/en/pdf-to-xrechnung?source=resource&bt=BT-73)

## In plain language

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.

## Where the field sits

BT-73 belongs to the Delivery part of the invoice. UBL 2.1 Syntax: Invoice › cac:InvoicePeriod › cbc:StartDate. CII (UN/CEFACT) Syntax: rsm:CrossIndustryInvoice › rsm:SupplyChainTradeTransaction › ram:ApplicableHeaderTradeSettlement › ram:BillingSpecifiedPeriod › ram:StartDateTime › udt:DateTimeString.

## What goes wrong often

Avoid values like: 01.04.2026, 2026/04/01

## Valid Examples

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

## Invalid Examples

- 01.04.2026
- 2026/04/01

## Related Fields

- [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

## XML Implementation

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

### CII (UN/CEFACT) Syntax
```
/rsm:CrossIndustryInvoice/rsm:SupplyChainTradeTransaction/ram:ApplicableHeaderTradeSettlement/ram:BillingSpecifiedPeriod/ram:StartDateTime/udt:DateTimeString
```

## Create an XRechnung from your PDF

The converter reads the invoice and shows the fields it found before download, so you can check and correct the values. No PDF? Fill in the XRechnung directly in the form.

- [Convert invoice](/en/pdf-to-xrechnung)
- [Fill in XRechnung in the form](/en/create-xrechnung)

Trying it and buying a single conversion work without an account. Subscriptions from €10/month with annual billing (excl. taxes), or €5 per conversion (excl. taxes) without a subscription.

## FAQ

### How should I state the service period on the original invoice?
Use an explicit label, for example “Service period: 01.09.2026 to 30.09.2026”. Check those dates in the review fields for the period start (BT-73) and end (BT-74). Do not infer them from the invoice date or due date. Clear wording helps recognition but does not guarantee it. The technical date-format examples describe structured data, not how dates must be printed on a PDF.

## Official sources and tools

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