On the Add or Edit Service overlay, there is a field called Invoice Time Format:
This feature is most commonly used for Snow services but it can be used on any Service. Use the dropdown list to make a selection of what timestamps you want to display on the Invoice: the start time, the end time, or both. If you modify this field, don't forget to Save your changes. You will have to record job times through one of the mobile apps or manually on the Close Out Day screen in order for the timestamp to generate on an Invoice.