Abstract: An electronic wristwatch having an LCD for time of day, day of month and month of year, as well as four key switches, includes a photodiode for detecting skin damaging ultraviolet, tanning radiation. The photodiode, a microcomputer, key switches, display and a battery are electrically connected to each other so that in response to activation of the key switches the microcomputer stores signals indicative of the time functions, intensity of the skin damaging ultraviolet radiation instantaneously incident on the photodiode at the time one of the key switches is activated, skin type of a subject wearing the watch, subject sun protection factor, and elapsed time from the time one of the key switches is activated, and the remaining time the subject skin can be safely exposed to the skin damaging radiation. The remaining time indication is derived independently of accumulated radiation incident on the photodiode which need not be pointed at the radiation source except when the one key is activated.