This is a Safari version of a Google Chrome extension created by maccman. GitHub Repo here. In order to add this extension to Safari, you need to modify the Date value containing your dob in app.js, and then build a new Motivation.safariextz extension. This requires an Apple Dev Account and a signed Safari Dev certificate.
(This extension is currently hard-coded to my birthday, due to issues with the date-picker not showing up in Safari.)
