# Period References

## Overview

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

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**](/data-types/data-types.md#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**](/calculations/the-calculation-page/the-formula-entry-bar.md#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="/files/6pTy8dvZAkWAhSLTwcyW" 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="/files/CsZ5JM8stT3uj5fZuW8f" 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**](/calculations/calculation-parts/variable-references/referencing-time.md) for details of how to edit these.&#x20;


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