Calendar and Countdown is a free Chrome extension which provides some simple calendar functions.
It displays a full-year calendar, with week numbers, and links to view the calendar for other recent years.
Click the date for some important event and you're able to give it a brief text description, a colour code. Set it as your Primary date and Calendar and Countdown's icon displays the number of days until it's due.
Click as many other dates as you like, add descriptions and colour coding to help group them.
Hovering your mouse over a date displays some useful stats: it's the nth day of the year, there are x days until the end of the year, it's y days from now, and z days until your primary date, and so on.
There are handy bonus functions everywhere you look. For example, click one date, hold and drag to another, and you're told how many days you've highlighted.
The extension also has an array of configuration options, from calendar and date formats, to your preferred week start day, icon styles, badge colours and more.
It's about as simple as any time management program can get, but Calendar and Countdown performs its few functions very well, it's extremely easy to use, and doesn't require any Chrome permissions at all.