Using a combination of tags, smart filters, and List view, you can generate a custom view that displays the total amount of time spent on a project for calculating billable hours, etc.
For example, to track the amount of time spent working on the xyz project each month, do the following:
Select List view, choose a Month date range from the pop-up menu at the top, and then Control-click the column header area and add these columns: Duration, Total Duration, and Tags.
Create a smart filter that remembers the current view settings and displays only events that contain the xyz tag.
Now, whenever you invoke this smart filter, it switches to List view, displaying only events that contain the xyz tag, with columns showing the Duration of each event and a running Total Duration for all events.
You can also show columns for Travel Time and Total Travel Time in the List View, if you want you want to track or bill your travel time separately.