How to compare two dates(dates only; not time) in cocoa?

Basically, I want to figure out if it's the next day. So, I'm storing the current date (e.g. Jan 2) constantly in开发者_开发技巧 a plist. But the next time the user opens the application, if the date has changed (e.g. Jan 3), I want to do something. Note that a simple ascending order check wouldn't work because I don't want to know if one date is later than another date, if the difference is only in hours. I need to be able to differentiate Jan 2 11:50 and Jan 3 2:34 but not Jan 3 2:34 and Jan 3 5:12.

I use the following that I found SO:

- (BOOL)isSameDay:(NSDate*)date1 otherDay:(NSDate*)date2 {
    NSCalendar* calendar = [NSCalendar currentCalendar];

    unsigned unitFlags = NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay;
    NSDateComponents* comp1 = [calendar components:unitFlags fromDate:date1];
    NSDateComponents* comp2 = [calendar components:unitFlags fromDate:date2];

    return [comp1 day]   == [comp2 day] &&
    [comp1 month] == [comp2 month] &&
    [comp1 year]  == [comp2 year];
}

Looking at the Date Time Programming Guide gives you an example of how to calculate the difference between two dates as by component: for example:

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];

My version based off Neils' (warning: untested):

Mine is more similar to - (NSComparisonResult)compare: on NSDate. To check for days being equal, do [myDate compareDayMonthAndYear:otherDate] == NSOrderedSame

- (NSComparisonResult)compareDayMonthAndYear:(NSDate *)date
{
    NSCalendar *calendar = [NSCalendar currentCalendar];

    unsigned unitFlags = NSYearCalendarUnit | NSMonthCalendarUnit |  NSDayCalendarUnit;
    NSDateComponents *selfComponents = [calendar components:unitFlags fromDate:self];
    NSDateComponents *dateComponents = [calendar components:unitFlags fromDate:date];

    BOOL dayIsEqual = (selfComponents.day == dateComponents.day && selfComponents.month == dateComponents.month && selfComponents.year == dateComponents.year);
    if (dayIsEqual) {
        return NSOrderedSame;
    }
    return [self compare:date];
}

Use NSCalendarDate instead of NSDate, and compare what they return from -dayOfCommonEra

@implementation NSDate (sameDayOrNot)

- (BOOL) sameDayAsDate:(NSDate *) otherDate
{
NSCalendarDate *date1 = [[NSCalendarDate alloc] initWithTimeInterval:0 sinceDate:self];
NSCalendarDate *date2 = [[NSCalendarDate alloc] initWithTimeInterval:0 sinceDate:otherDate];
BOOL result = ([date1 dayOfCommonEra] == [date2 dayOfCommonEra]);
[date1 release];
[date2 release];
return result;
}

@end