Comparing two NSDates and ignoring the time component

Question

What is the most efficient/recommended way of comparing two NSDates? I would like to be able to see if both dates are on the same day, irrespective of the time and have started writing some code that uses the timeIntervalSinceDate: method within the NSDate class and gets the integer of this value divided by the number of seconds in a day. This seems long winded and I feel like I am missing something obvious.

The code I am trying to fix is:

if (!([key compare:todaysDate] == NSOrderedDescending))
{
    todaysDateSection = [eventSectionsArray count] - 1;
}

where key and todaysDate are NSDate objects and todaysDate is creating using:

NSDate *todaysDate = [[NSDate alloc] init];

Regards

Dave

Answer 1

I'm surprised that no other answers have this option for getting the "beginning of day" date for the objects:

[[NSCalendar currentCalendar] rangeOfUnit:NSCalendarUnitDay startDate:&date1 interval:NULL forDate:date1];
[[NSCalendar currentCalendar] rangeOfUnit:NSCalendarUnitDay startDate:&date2 interval:NULL forDate:date2];

Which sets date1 and date2 to the beginning of their respective days. If they are equal, they are on the same day.

Or this option:

NSUInteger day1 = [[NSCalendar currentCalendar] ordinalityOfUnit:NSDayCalendarUnit inUnit: forDate:date1];
NSUInteger day2 = [[NSCalendar currentCalendar] ordinalityOfUnit:NSCalendarUnitDay inUnit:NSCalendarUnitEra forDate:date2];

Which sets day1 and day2 to somewhat arbitrary values that can be compared. If they are equal, they are on the same day.

Answer 2

You set the time in the date to 00:00:00 before doing the comparison:

unsigned int flags = NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay;
NSCalendar* calendar = [NSCalendar currentCalendar];

NSDateComponents* components = [calendar components:flags fromDate:date];

NSDate* dateOnly = [calendar dateFromComponents:components];

// ... necessary cleanup

Then you can compare the date values. See the overview in reference documentation.

Answer 3

There's a new method that was introduced to NSCalendar with iOS 8 that makes this much easier.

- (NSComparisonResult)compareDate:(NSDate *)date1 toDate:(NSDate *)date2 toUnitGranularity:(NSCalendarUnit)unit NS_AVAILABLE(10_9, 8_0);

You set the granularity to the unit(s) that matter. This disregards all other units and limits comparison to the ones selected.