App feature works on 64-bit devices but not 32-bit i.e. iPhone 5 and earlier

My app displays a school timetable for the whole year, the user selects a date then the app takes data from a JSON file that corresponds to that date. This data includes details such as the name of the class and the start and finish times. Each class is stored as a separate object then put in to an array. The start and finish times are of type NSDate. From here it is eventually drawn out on to the screen in the correct order.

Before it is drawn out i have an algorithm that checks if any of the events have clashing times. If it finds any that do, they are drawn on screen in a different colour to draw attention to this.

Now when i run this on any device using 64-bit( like the 5s, 6 or the new iPads), this works exactly as it is supposed to. When i run it on a 32-bit device (iPhone 5 and earlier) it goes haywire, it signifies clashes on the classes at random and this changes every time i run the app.

I'm not looking for help with my specific case but i wanted to know if there are any major differences that i need to watch out for and any changes that id need to make when trying to develop for both sets of devices?