Now this is a nasty one.  If you are not running IOS 9.3.1 an automated WiFi based attack against your Idevice(which is not that hard to carry out) can permanently brick your expensive Idevice.  The problem is that if your Idevice gets set back to the date of Jan 1, 1970 then the device will freeze and possibly be bricked forever.    Short of extremely technical means there is no recovering the Idevice.  The bad part is now two other security researchers have figured out a way to trick your Idevice to hook into a wifi network it has seen before.  Once that happens the Idevice checks for time via NTP..the wifi network can make this one of their time servers and tell the device it is Jan 1, 1970 therefore killing your Idevice.  If you have not(or if your device is very old cannot) update you need to do so as soon as you can.  If your device is out of support then you run this risk of one day your device just freezing.