Objective-C NSDate example of checking how many times a day-of-the-month appears between two dates. See: http://stackoverflow.com/questions/21161125/how-many-times-a-specific-day-has-passed-between-two-dates Compile with: clang -g -o dateiter dateiter.m -fobjc-arc -framework Foundation

dateiter.m

#import <Foundation/Foundation.h>

static void nextMonth(NSDateComponents *comps) {
    NSInteger month = [comps month];
    if (month == 12) {
        [comps setYear:[comps year] + 1];
        [comps setMonth:1];
    } else {
        [comps setMonth:month + 1];
    }
}

int main(int argc, const char **argv) {
    @autoreleasepool {
        if (argc != 4) {
            NSLog(@"usage: %s start-date end-date want-day", argv[0]);
            NSLog(@"date format: yyyyMMdd");
            return 1;
        }

        NSTimeZone *tz = [NSTimeZone timeZoneWithAbbreviation:@"UTC"];
        NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
        [calendar setTimeZone:tz];
        NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
        [formatter setTimeZone:tz];
        [formatter setCalendar:calendar];
        [formatter setDateFormat:@"yyyyMMdd"];

        NSDate *startDate = [formatter dateFromString:[NSString stringWithUTF8String:argv[1]]];
        if (!startDate) {
            NSLog(@"Invalid start-date '%s'", argv[1]);
            return 2;
        }

        NSDate *endDate = [formatter dateFromString:[NSString stringWithUTF8String:argv[2]]];
        if (!endDate) {
            NSLog(@"Invalid end-date '%s'", argv[2]);
            return 3;
        }

        char *endptr;
        NSInteger wantDay = (NSInteger)strtol(argv[3], &endptr, 10);
        if (wantDay <= 0 || wantDay > 31 || *endptr != '\0') {
            NSLog(@"Invalid want-day '%s'", argv[3]);
            return 4;
        }

        NSLog(@"start-date=%@, end-date=%@, want-day=%ld", startDate, endDate, (long)wantDay);

        const NSCalendarUnit units = NSEraCalendarUnit|NSYearCalendarUnit|NSMonthCalendarUnit|NSDayCalendarUnit|NSCalendarCalendarUnit|NSTimeZoneCalendarUnit;
        NSDateComponents *comps = [calendar components:units
                                              fromDate:startDate];
        if ([comps day] > wantDay)
            nextMonth(comps);       // Missed the first month
        [comps setDay:wantDay];

        NSInteger count = 0;
        while (YES) {
            NSDate *compareDate = [calendar dateFromComponents:comps];
            if (!compareDate) {
                NSLog(@"Failed to create compare date");
                return 5;
            }
            NSLog(@"Comparing %@", [formatter stringFromDate:compareDate]);
            if ([compareDate compare:endDate] == NSOrderedDescending)
                break;
            count++;
            nextMonth(comps);
        }

        NSLog(@"%ld months", (long)count);
    }
    return 0;
}