While no one should really stay out in direct summer sun too long, and certainly not without sun protection, this is a mobile app that details for you exactly how long you can be out before your skin starts to get a bit toasty.
Called Sun Exposure by Wolfram Alpha, you either let the app find your location or you can out one in, put in the time of day you'll be outside, your approximate skin type, and it'll spit out for you how long you have before burning with no sun protection but also wearing SPF 15, 30, and 45. It also predicts for you the UV index and tells you sun position, plus sunrise and sunset times. Sun Exposure is an iOS app for 99 cents.
And here's a similar app only designed to tell on hot summer days when you should step inside because the air is becoming less safe to breathe.
Put out by the American Lung Association, it's called State of the Air. Either let it discover where you are or input a location, and it serves up ozone and particulate information of the day along with a color to indicate how safe that number is. Green is the safest while red is the most dangerous. You can tap the number for more information.
The app also offers to put you on email lists to stay up to date with air quality issues and gives you a one-touch button to contact lawmakers in order to voice your concern about the air quality. State of the Air is a free app for iOS and android devices.