I want to create an iphone app to ensure that my children are safe in her hotel/home before I go to sleep at night. One resides in America (UTC-6.00) and the other is in Athens (UTS+2.00). I'm in Australia (UTC +10) = 14 Hours apart. The idea is, they will press a button on their Iphone app before they go to sleep at 10pm their time, every night. So its 10pm, they are about to sleep, they press the button. Before I go to sleep in Australia, I press a "check" button to see if they have checked in so to speak. What is the algorithm that I need to work out for the app so that I know for sure that they have activated the button within the last 24 hours? Do I measure it against the UTC, so +12 and -13? Any help would be much appreciated.