The relative time plugin should be able to do what you need. Have you looked at the demo and tried to follow the instructions on the plugin page? If so and you’re still having trouble, please share details of where you’re stuck exactly, perhaps with some screenshots or a link to your editor.
Quick tip - in your case I think you need to set the target date of the relative time plugin element to the (user’s) launch date, then in the text field that you want to show the relative time (like ‘18 days ago’), set it’s value to be the relative time element’s current value.