http://iphonedevelopment.blogspot.com/2009/07/category-on-nsdate.html

NSDate-Misc.h

#import <Foundation/Foundation.h>
@interface NSDate(Misc)
+ (NSDate *)dateWithoutTime;
- (NSDate *)dateByAddingDays:(NSInteger)numDays;
- (NSDate *)dateAsDateWithoutTime;
- (int)differenceInDaysTo:(NSDate *)toDate;
- (NSString *)formattedDateString;
- (NSString *)formattedStringUsingFormat:(NSString *)dateFormat;
@end

NSDate-Misc.m

#import "NSDate-Misc.h"

@implementation NSDate(Misc)
+ (NSDate *)dateWithoutTime
{
    return [[NSDate date] dateAsDateWithoutTime];
}
-(NSDate *)dateByAddingDays:(NSInteger)numDays
{
    NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
    
    NSDateComponents *comps = [[NSDateComponents alloc] init];
    [comps setDay:numDays];
    
    NSDate *date = [gregorian dateByAddingComponents:comps toDate:self options:0];
    [comps release];
    [gregorian release];
    return date;
}
- (NSDate *)dateAsDateWithoutTime
{
    NSString *formattedString = [self formattedDateString];
    NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
    [formatter setDateFormat:@"MMM dd, yyyy"];
    NSDate *ret = [formatter dateFromString:formattedString];
    [formatter release];
    return ret;
}
- (int)differenceInDaysTo:(NSDate *)toDate
{
    NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
    
    NSDateComponents *components = [gregorian components:NSDayCalendarUnit
                                                fromDate:self
                                                  toDate:toDate
                                                 options:0];
    NSInteger days = [components day];
    [gregorian release];
    return days;
}
- (NSString *)formattedDateString
{
    return [self formattedStringUsingFormat:@"MMM dd, yyyy"];
}
- (NSString *)formattedStringUsingFormat:(NSString *)dateFormat
{
    NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
    [formatter setDateFormat:dateFormat];
    NSString *ret = [formatter stringFromDate:self];
    [formatter release];
    return ret;
}
@end