On the South corner of the castle buildings there is a turret with a sundial carved into it. The sundial has the words "Latet Ultima" and "Cursum Perfico" which translates to "the last is hidden" and "I stay the course".
If you look, the III is missing representing three oclock. This is where the Latet Ultima connection comes in, basically if no time is showing and its daylight then its more likely to be 3pm.
The turret is also home to the Hunterston Bees, if you click the link shown as turret bees you can read more about this.
The sundial used to keep pretty good time but in recent years with the growth of trees the time has got less accurate due to the sunlight being sheilded by branches and leaves of the neighbouring beech trees.