> For the complete documentation index, see [llms.txt](https://help.taglo.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://help.taglo.io/calculations/calculation-parts/period-references.md).
# 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
symbol)
2. Add a new Calculation Part, and select `Period` from the part type options
3. Use the **Periods** 
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.