Finding the rolling sum of a variable can allow you to:

- Calculate the headcount by getting the rolling sum of new hires
- Track the user count of an application by using the rolling sum of the new users etc.

The `sum`

function, span time modifiers and the `timeStep`

helper variable are the key tools that should help us accomplish this.

As the name suggests, the `sum`

function returns the sum of the parameters passed.

The `timeStep`

helper, also known as `t`

or `date`

returns the column number of a given cell.

If we write something like `sum(Variable[0:t])`

we should get the rolling sum of the variable.

We may also find the need to find periodic rolling sum. A use case would be to find the YTD Revenue. To implement this, we can get creative with the helper variables we and plug an appropriate expression in the time modifier. A formula similar to `sum(Revenue[ t-month+1 : t] ) `

should do the trick. The `month`

helper returns the month for the current column (i.e. 1 for January, 2 for February…) which is why writing ` t-month+1 : t`

in the time modifier should give us the span of the revenue from the last January to the current month.