Adding date and time to a photo

Do you want to write the date and time into the photo?

You need to do that in an external editor. Photos does not support it.

You can print the date and time and other captions onto a contact sheet, however. When youu select some photos and then use the "Contact Sheet" template, you can click the "Captions" button and select metadata to be used as the caption on the contact sheet.

To write directly into the photo, you can use the Preview.app.

Either export the photo from Photos and then open it in Preview, or install the "External Editors" extension from the App Store. Then you can open Preview directly from the Edit Panel in Photos. See this User Tip: External Editors in Photos Are Here

You're welcome, Christiana!