Not sure what you're getting at, but most upgraded camera apps have this option. I've been exploring the "Open Camera" app, which not only has a time/date stamp, but allows for custom text and GPS stamped images.. So if you're into just date/time stamping.. common apps are just ok.. If you want more options, best look into the Open Camera App by Mark Harman
I have application named Auto Stamper for Photos and it worked great for me. And also allows to use smartphone's default camera for all the stamps that no other application will allow you.
The Application has Three main features : Date and Time,Signature & GPS( Location)
If you can't get all the features on your photo then first you must look at the discription about all the features how to use while capturing the photo. So first, have a look about the application and then make use of it. You will get the nice result as your expectation.
One more app to constantly run in the background? No thanks! I too use Open Camera and only when I need to date stamp a photo. Other than that, I just go unstamped with the stock camera app.