When you are creating an invoice and want to add the time of that service, simply click the "extra fields" section (the 3 horizontal bars next to the "Add" button).

Once you are in this section, you will have the option of adding the Service Start Time, Service End Time and Service To Date. 

The "service to date" would be used if you were to for example start a service at 11:00pm on the 1st and the service goes until 1:00am on the 2nd, the service to date would be 02.