Make a new NSDate by combining two NSDates

Question

I have NSDate* day1 and NSDate* day2. How can I make a NSDate* day3 by copying only year/month/day from day1 and hour/minute/second from d

Answer 1

For the records, here is a more general approach

@implementation NSDate (Additions)

- (void)assignFlags:(NSUInteger)unitFlags to:(NSDateComponents *)components {
  NSDateComponents *myComponents = 
        [[NSCalendar currentCalendar] components: unitFlags
                                        fromDate: self];
  if (unitFlags & NSYearCalendarUnit)
    [components setYear: [myComponents year]];
  if (unitFlags & NSMonthCalendarUnit)
    [components setMonth: [myComponents month]];
  if (unitFlags & NSDayCalendarUnit)
    [components setDay: [myComponents day]];
  if (unitFlags & NSHourCalendarUnit)
    [components setHour: [myComponents hour]];
  if (unitFlags & NSMinuteCalendarUnit)
    [components setMinute: [myComponents minute]];
  if (unitFlags & NSSecondCalendarUnit)
    [components setSecond: [myComponents second]];
}

- (NSDate *)dateByTakingComponents:(NSUInteger)unitFlags fromDate:(NSDate *)date {

  NSUInteger allFlags = NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit |
    NSHourCalendarUnit | NSMinuteCalendarUnit | NSSecondCalendarUnit;

  NSDateComponents *allComponents = [[NSDateComponents alloc] init];
  [date assignFlags: unitFlags to: allComponents];
  [self assignFlags: allFlags & ~unitFlags to: allComponents];

  NSDate *result = [[NSCalendar currentCalendar] dateFromComponents: allComponents];
  [allComponents release];

  return result;
}

@end

You can call it like this

NSDate *day3 = 
    [day2 dateByTakingComponents: NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit
                        fromDate: day1];