> For the complete documentation index, see [llms.txt](https://help.modelreef.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://help.modelreef.io/syntax/timing-syntax/start-and-end-dates.md).

# Start & End Dates

This article explains **Start and End dates** in the Timing modal.

You will learn:

* How start and end dates bound accruals.
* How they interact with frequency and schedules.
* How changes to dates affect historical and forecast periods.

Start and end dates define the active window for a variable's accrual behaviour.

{% stepper %}
{% step %}

### Where start and end dates are set

You set these dates in the **Timing modal** for each variable. For imported variables, they may be prefilled based on the imported series.

The fields usually include:

* A **start date** calendar or picker.
* An **end date** calendar or picker, which can be open ended in some configurations.
  {% endstep %}

{% step %}

### How start dates work

The start date is the first date on which the variable can accrue value.

* Before the start date, accrual is zero.
* On and after the start date, accrual follows the chosen frequency and schedule.

In models with historical data:

* The start date may align with the first historical period.
* Or it may be set to the first forecast period for new items.
  {% endstep %}

{% step %}

### How end dates work

The end date is the last date on which the variable accrues.

* After the end date, accrual drops to zero.
* Cash may still occur after the end date if delays are present.

End dates are important for:

* Fixed term contracts.
* Loan or lease maturities.
* Time limited projects or campaigns.
  {% endstep %}

{% step %}

### Interaction with model start and end dates

The model itself has a start date and end date. Variable start and end dates are interpreted within that window:

* If a variable's start date is before the model start date, accrued history before the model is usually represented only via opening balances.
* If a variable's end date is after the model end date, only the portion within the model horizon is simulated.

Adjusting model dates may change which parts of a variable's life are visible.
{% endstep %}

{% step %}

### Editing dates safely

When editing start and end dates:

* Consider whether the change is a structural correction or a new business assumption.
* For historical periods tied to imports, reducing the visible window will not modify the source data, only which part is used.
* For future periods, extending or shortening the window alters the forecast horizon for that variable.

Use notes to document large date changes so reviewers understand the intent.
{% endstep %}
{% endstepper %}

## Related articles

* [Allowed Operators](/syntax/formula-syntax/allowed-operators.md)
* [Auto-Validation Rules](/syntax/how-input-fields-work/auto-validation-rules.md)
* [Cleaning Headers](/help/importing-and-data-inputs/cleaning-headers.md)
* [Convert a PDF to a Structured Forecast](/how-tos/data-workflows-and-automation/convert-a-pdf-to-a-structured-forecast.md)