Make a Timestamp when single field is modified

I want to add a 'modified on' date field for some custom fields. I want it to record the last date a specific field was altered and saved.

I tried using a business rule but it didn't work how I wanted. It just shows the current date. I want the time to only reflect the date it got modified, and remains that date until the field is modified again. A timestamp like what's on the notes section would be good. I just need the date not time. 

I used this as my business rule:

Does anyone know how to do this? would it be a workflow? is there a step by step tutorial somewhere, I'm not sure how to enter a date variable into it. A date based on the current date, but not keep changing as days pass.

Thanks