timesince.link is a lightweight web tool (about 16KB) for measuring the time since a certain event on a certain date. The link contains all the info about your event so you can bookmark it, send it to people, or whatever else.
It's hosted on static web hosting so no data is sent back to a server. The only place your data is stored is inside the link you generate. The javascript on the landing page simply reads your link and calculates the time. Very clean if you ask me.