# Period References

## Overview

If you need to use a date from your Model's [**Timings**](https://help.taglo.io/models/home-tab/creating-a-model/timings) in a [**Calculation**](https://help.taglo.io/calculations), you can do this using a Period Reference [**Calculation Part**](https://help.taglo.io/calculations/calculation-parts).

When you reference a period of the Model, the value returned is a representation of the number of days since 1st January 1900 at the end of the period, as explained in the [**Data Types documentation**](https://help.taglo.io/data-types/data-types#datetime-date-treatments), equivalent to the treatment of dates in Excel.

> For instance, if you want to calculate the number of days in a period of the Model, you would use the Calculation `Periods - Period [-1]`. This subtracts the date value at the end of the previous period from the date value at the end of the current period.

## Creating Period References

There are 3 ways of creating a Period Reference:

1. Use the [**AutoComplete dropdown**](https://help.taglo.io/the-calculation-page/the-formula-entry-bar#full-list-of-autocomplete-option-types) in the Formula Entry Bar (type "Periods" and select the Periods item as indicated by the clock<img src="https://2847533834-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FVy65sax6FVoNo2UTlJ0x%2Fuploads%2FOLGmxk5yoRDa06Dzmn7D%2Fimage.png?alt=media&#x26;token=a58c7b14-9bab-4ae1-bae3-f65e7de60092" alt="" data-size="line">symbol)
2. Add a new Calculation Part, and select `Period` from the part type options
3. Use the **Periods** <img src="https://2847533834-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FVy65sax6FVoNo2UTlJ0x%2Fuploads%2F2nWSVovEi4pcPmC7KIFB%2Fimage.png?alt=media&#x26;token=975e87c2-3a8f-41d9-8692-986c0ccbaad5" alt="" data-size="line"> shortcut button which can be found at the bottom of the Calculation Part list

## Editing Period References

Periods References can reference single or multiple time periods, and reference offset periods, similarly to Variable References.

See the [**Variable References documentation**](https://help.taglo.io/calculations/calculation-parts/variable-references/referencing-time) for details of how to edit these.&#x20;