Circuit allows you to easily specify if you need to arrive at a stop within a certain time-window.
If set, Circuit will adjust the rest of the route to ensure you arrive on-time, or slightly before the specified window.
Here's how to do it:
Add your stops to the route in the usual way.
On the "Edit Route" screen, tap the menu button (3 vertical dots) associated with the stop you want to attach a time-window too, to open the menu.
From here, you can set the stop's time-window.
Finish editing, and optimise the route as normal.
When using time-windows, you must optimize the route just before you leave. If you've already optimized the route, you can "reoptimize" using the menu button in the top-right of the screen.