> For the complete documentation index, see [llms.txt](https://swatplus.gitbook.io/io-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://swatplus.gitbook.io/io-docs/introduction-1/simulation-settings/time.sim/day_start.md).

# day\_start

Beginning day of the simulation

SWAT+ is able to begin a simulation at any day of the year. This option can for example be useful, if the user wishes to simulate hydrological years instead of calendar years.

{% hint style="info" %}
If *day\_start* = 0, the model will start the simulation on January 1st.

If the simulation begins before the first day of the observed climate data, SWAT+ will use simulated climate data for the time period before the start of the observed climate data.
{% endhint %}