"On the invoices I want to show my clients the dates as well as the hours spent for a client."
I don't think any of the choices are very good.
If any of your invoices contains work done on multiple dates, I believe the only way is to have one invoice line for each date being billed and use the Item Description field to record the date.
For invoices that contain only work done on one specific date, you could use the Customer Message; but since each of those messages is saved, that might not work out too well. Alternatively; if you don't already use the P.O.
Number field, you could repurpose that field to hold your Date of Service: the P.O. Number field can contain 12 characters. [To give an invoice field a new heading, see Business > Invoices and Estimates > Design invoice forms.]