How do I know what time it is?

There are various events in the Witcher 2 which requires me to drop by a location by, say, 21:00hr.

How do I know what it is now in the game? There's an icon next to the radar, but it doesn't give the exact time.

You can only see the exact time when you're meditating. Just press Ctrl and click in the middle of the radial menu to meditate. In the following menu click again on meditate and you'll get a display that shows you the time. Here you can also wait until a specific time, which is very useful if you need to be somewhere at a specific time.

You can see a rough estimate of the time in your top right corner of the screen. The small circle is actually a clock, you can see the small mark on the outside of the circle indicating the time. In this screenshot it is somewhere around 10 o'clock.