# Working with time

The key concept to understand about working with time in Causal - is Causal's **time step index system**.

Note that the time step index always start at 0, not 1. So 0 corresponds to the first time step of the model (e.g. Jan'22), 3 corresponds to the fourth time step (Apr'22), and so on.

This time step index allows you to modify variables in formulas, e.g. refer to the value of a variable in three months. See Time modifiers for more.

Causal also has a bunch of in-built helper variables (like date, month, year, today, lastActualDate etc) that make it easy to work with time. See our common formulas on Time Dependent formulas here.

### Dates in Causal

The `Date`

function in Causal converts an input in `Date`

format to the appropriate timestep in that model to allow further calculations.

The syntax of the `Date`

function is as follows: either (yyyy,mm,dd) or (yyyy,mm).

E.g. date(2022,12,24) = 11 in a monthly model that starts in Jan 2022.