The intensity of UV radiation reaches its maximum at midday. When the sun is directly overhead solar radiation has to pass through the least amount of protective atmosphere, allowing large amounts of UV light to reach the surface of the earth. So be smart and seek shade between 11 a.m. and 3 p.m. when the atmosphere provides the least amount of protection. Staying under a sun-shade will reduce the amount of UV radiation shining down on you significantly. Shade will not only protect you from UV it will also keep you cool.
There is an easy rule to remember:
If your shadow is shorter than you are tall, stay out of the sun. This rule applies because when the sun reaches its highest orbit point it will shine straight down on you. Your shadow will be very short. The sun will reach this point around noon.