What hours of the day are the rays of the sun the deadliest?

The Sun's rays are capable of causing sunburn, and constant long-term exposure can lead to an increase in skin cancers. That said, sunlight is only "deadly" to vampires.

From about 11 AM to 3 PM (daylight savings time) is when sunlight is most direct, and when the atmosphere provides less protection. During morning and evening hours, the Sun's rays are filtered somewhat by atmospheric scattering and attenuation.