This is really important and it has not been discussed yet. No matter if the model you choose is coated to be UV-resistant or simply naturally is, you should avoid leaving it in direct sunlight. This is not even a matter of damage to the product, either. Instead, it all comes down to the accuracy if the model also measures temperature and humidity levels.
When a temperature sensor is being beamed on by the sun, it is going to give off inaccurate readings of the actual ambient temperature. If it is actually 80 degrees outside, it may say it is 110 degrees but it is baking in the sun. If you can, try and mount your outdoor clock in an area that experiences mostly shade (if you care at all about the accuracy of the other sensors).