The time period allows you to filter your data by its primary time slice (when the event occurred). The time period is the first pill in the row of filters.

You can change the value to a custom range by choosing how you'd like to use the time period.

Then, once you choose how you'd like to define your time period, choose the values to complete the time period selection and click off the popover.

Click 'Run Question' to apply your changes.

You can also add a comparison time period in the date filter to visualize period over period analysis.