Iphone NSDate and NSDateComponents

 

http://developer.apple.com/iphone/library/documentation/Cocoa/Conceptual/DatesAndTimes/Articles/dtCalendars.html#//apple_ref/doc/uid/TP40003470-SW1

1. use components:fromDate:toDate:options: to determine the temporal difference between two dates in units other than seconds (which you could calculate with the NSDate method timeIntervalSinceDate

NSDate *startDate = ...; 
NSDate *endDate = ...; 
  
NSCalendar *gregorian = [[NSCalendar alloc] 
                 initWithCalendarIdentifier:NSGregorianCalendar]; 
  
NSUInteger unitFlags = NSMonthCalendarUnit | NSDayCalendarUnit; 
  
NSDateComponents *components = [gregorian components:unitFlags 
                                          fromDate:startDate 
                                          toDate:endDate options:0]; 
NSInteger months = [components month]; 
NSInteger days = [components day]; 

2. use the dateByAddingComponents:toDate:options: method to add components of a date (such as hours or months) to an existing date

NSDate *today = [[NSDate alloc] init]; 
NSDateComponents *offsetComponents = [[NSDateComponents alloc] init]; 
[offsetComponents setHour:1]; 
[offsetComponents setMinutes:30]; 
// Calculate when, according to Tom Lehrer, World War III will end 
NSDate *endOfWorldWar3 = [gregorian dateByAddingComponents:comps toDate:today options:0];