There are two different ways to record time in Workstack.
The Time log
You can access the Time Log of each Todo by opening it's Edit panel and clicking the stopwatch icon.
The Live Recorder lets the logged in user record their time as they go. Just click the context menu icon inside a task on the calendar and then click the stopwatch icon.
This will then show the live recorder widget at the bottom-right.
To start recording time just click the widget.
When you're finished just click the widget again and then click the stopwatch icon that appears - this will then open the Time Log with the amount of time worked already entered, just click Save to add this to the log.
Note: The live recorder needs to be running for at least 5 minutes before anything will appear in the Time Log. The Time Log will round the time worked to the nearest decimal - so 5 mins will be logged as 0.1 hrs.