# Built-in variables

OpenAgua includes a growing list of built-in variables available for use in functions. As of writing, these include the following:

### `timestep`

An ordinal integer representing the current timestep (i.e., 1, 2, 3, ..., N).

### `date`

A the current timestep's date as a [Pendulum](https://pendulum.eustace.io) date object, which has numerous useful [attributes and properties](https://pendulum.eustace.io/docs/#attributes-and-properties) (e.g., `date.month`, etc.).

### `periodic_timestep`

An ordinal integer representing the timestep in the current water year. For example, a monthly time step model starting in October would have `periodic_timestep` of 1, 2, 3, ..., 12, with 1 in October and 12 the following September. A daily time step model would similarly range from 1 to 365 (or 366 if leap years are considered).

### `water_year`

The current water year, which is defined by the calendar year of the *last month* in the water year. If it helps, here is the specific equation for `water_year`:

```python
water_year = date.year + (0 if date.month < start_date.month else 1)
```

where `start_date` is the start date of the model. Speaking of start date...

### `start_date`

This is a [Pendulum](https://pendulum.eustace.io) date object representing (you guessed it!) the start date of the model.

{% hint style="info" %}
This list is preliminary, and we would happily accommodate additional variable needs.
{% endhint %}