Source code can be found here: https://github.com/aquilax/uptime-chrome . Feel free to remix and modify.
# Changelog
v0.4 Switched to using the more stable chrome.storage.local API to store the startup time.
v0.3 Switched back to non-persistent version. Now the extension won't consume memory and will only on startup and when the icon is clicked;
v0.2 Switched to persistent extension to insure the initial time snatch