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# Line item: anchor dates

## Line item: anchor dates

This page explains what decides the day on which the billing periods of a line item start.

### Map of concepts

| Concept | Details |
| - | - |
| [Anchor date](#anchor-dates-where-billing-periods-start) | Without an anchor date, billing periods start on the start date of the line item. With an anchor date, they start on the anchor date and every billing cadence before and after it. |
| [Last-day-of-month alignment](#last-day-of-month-alignment) | If the anchor date is the last day of its month, last-day-of-month alignment starts every billing period on the last day of its month. |
| [Several anchor dates](#several-anchor-dates) | A line item can change its anchor date partway through, with a separate anchor date for each range of dates. |

### Related modules

Anchor dates uses the following modules:

| Module | Details |
| - | - |
| [Contracts](/docs/20260301/product-modules/contracts/concepts/contracts) | Anchor dates belong to the line items of a contract. |

## Anchor dates: where billing periods start

An **anchor date** is the date from which Zenskar counts the billing periods of a line item, within a range of dates.

Without an anchor date, the [billing periods](/docs/20260301/product-modules/contracts/glossary#billing-period) of a [line item](/docs/20260301/product-modules/contracts/glossary#line-item) start on the start date of the line item. The UI shows this as **Default**. For example, a line item starts on 10 March with monthly billing. Its billing periods run from 10 March to 9 April, from 10 April to 9 May, and so on.

With an anchor date, the billing periods start on the anchor date, and on every [billing cadence](/docs/20260301/product-modules/contracts/glossary#billing-cadence) before and after it. The time between the start of the line item and the first such date is a partial billing period.

For example, a line item starts on 1 January with monthly billing. Its anchor date is 15 January:

| Billing period | Length |
| - | - |
| 1 January to 14 January | Partial |
| 15 January to 14 February | Full |
| 15 February to 14 March | Full |

The same applies to other billing cadences. With quarterly billing and an anchor date of 1 February, the billing periods are 1 January to 31 January (partial), 1 February to 30 April, 1 May to 31 July, and so on.

With a monthly or longer billing cadence, each billing period starts on the same day of the month as the anchor date. When a month is shorter, the billing period starts on the last day of that month, and the next one goes back to the day of the anchor date. For example, an anchor date of 31 January gives billing periods that start on 28 February, 31 March, 30 April, and 31 May.

## Last-day-of-month alignment

**Last-day-of-month alignment** is an option of an anchor date that starts every billing period on the last day of its month.

Last-day-of-month alignment takes effect only when the anchor date is the last day of its month, such as 31 January, 28 February, or 30 April. For example, an anchor date of 28 February gives these billing periods:

| Last-day-of-month alignment | Billing periods start on |
| - | - |
| Off | 28 February, 28 March, 28 April |
| On | 28 February, 31 March, 30 April |

The UI offers last-day-of-month alignment for every anchor date on the 28th or later. For an anchor date that is not the last day of its month, such as 30 January, last-day-of-month alignment has no effect.

## Several anchor dates

A line item can have several anchor dates, each for its own range of dates. The ranges cannot overlap, and they must stay within the [line-item duration](/docs/20260301/product-modules/contracts/glossary#line-item-duration). Each anchor date falls within its own range.

A new anchor date applies from the start of its range. The last billing period of the earlier range ends with that range.

For example, a line item runs for the year with monthly billing. The anchor date is 15 January from January to June, and 1 July from July to December:

| Billing period | Anchor date |
| - | - |
| 15 May to 14 June | 15 January |
| 15 June to 30 June | 15 January (partial) |
| 1 July to 31 July | 1 July |
| 1 August to 31 August | 1 July |


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