iOS/iPhone DateDiff / TimeDiff / Convert Date (NSString to NSDate, NSDate to NSString)

    NSString *string1 = @"2012-12-05";
    NSString *string2 = @"2012-12-09";
    
    // Date Format
    NSDateFormatter *dateFormater = [[NSDateFormatter alloc] init];
    [dateFormater setTimeZone:[NSTimeZone timeZoneWithName:@"Asia/Bangkok"]];
    [dateFormater setDateFormat:@"YYYY-MM-dd"];
    
    NSDate *datetime1 = [dateFormater dateFromString:string1];
    
    NSDate *datetime2 = [dateFormater dateFromString:string2];
    
    double dateInterval = [datetime2 timeIntervalSinceDate:datetime1] / (60*60*24);
    
    NSMutableString *resultDateDiff = [NSString stringWithFormat:@"DateDiff = %.0f Days",dateInterval];
    lblDateDiff.text = resultDateDiff;

    NSString *string1 = @"2012-12-09 15:45:00";
    NSString *string2 = @"2012-12-09 15:50:00";
    
    // Date Format
    NSDateFormatter *dateFormater = [[NSDateFormatter alloc] init];
    [dateFormater setTimeZone:[NSTimeZone timeZoneWithName:@"Asia/Bangkok"]];
    [dateFormater setDateFormat:@"YYYY-MM-dd HH:mm:ss"];
    
    NSDate *datetime1 = [dateFormater dateFromString:string1];
    
    NSDate *datetime2 = [dateFormater dateFromString:string2];
    
    double timeInterval = [datetime2 timeIntervalSinceDate:datetime1] / (60);
    
    NSMutableString *resultTimeDiff = [NSString stringWithFormat:@"TimeDiff = %.2f Minutes",timeInterval];
    lblTimeDiff.text = resultTimeDiff;

    NSDateFormatter *dateFormater = [[NSDateFormatter alloc] init];
    [dateFormater setDateFormat:@"YYYY-MM-dd HH:mm:ss"];
    
    // NSString to NSDate
    NSString *dateString = @"2012-12-09 15:45:00";
    NSDate *dateFromString = [[NSDate alloc] init];
    dateFromString = [dateFormater dateFromString:dateString];
    
    // NSDate to NSString
    NSString *strDate = [dateFormater stringFromDate:[NSDate date]];
    lblConvertDate.text = strDate;

    NSDateFormatter *dateFormater = [[NSDateFormatter alloc] init];
    [dateFormater setDateFormat:@"YYYY-MM-dd HH:mm:ss"];
    
    // NSString to NSDate
    NSString *dateString = @"2012-12-09 15:45:00";
    NSDate *dateFromString = [[NSDate alloc] init];
    dateFromString = [dateFormater dateFromString:dateString];
    
    // NSDate to NSString
    NSString *strDate = [dateFormater stringFromDate:[NSDate date]];
    lblConvertDate.text = strDate;

//
//  ViewController.h
//  DateDiffTimeDiff
//
//  Created by Weerachai on 12/8/55 BE.
//  Copyright (c) 2555 Weerachai. All rights reserved.
//

#import <UIKit/UIKit.h>

@interface ViewController : UIViewController
{
    
    IBOutlet UILabel *lblDateDiff;
    
    IBOutlet UILabel *lblTimeDiff;
    
    IBOutlet UILabel *lblConvertDate;
    
}

@end

//
//  ViewController.m
//  DateDiffTimeDiff
//
//  Created by Weerachai on 12/8/55 BE.
//  Copyright (c) 2555 Weerachai. All rights reserved.
//

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
	// Do any additional setup after loading the view, typically from a nib.
    
    self.DateDiff;
    self.TimeDiff;
    self.ConvertDate;
    
}

// DateDiff
- (void)DateDiff
{
    NSString *string1 = @"2012-12-05";
    NSString *string2 = @"2012-12-09";
    
    // Date Format
    NSDateFormatter *dateFormater = [[NSDateFormatter alloc] init];
    [dateFormater setTimeZone:[NSTimeZone timeZoneWithName:@"Asia/Bangkok"]];
    [dateFormater setDateFormat:@"YYYY-MM-dd"];
    
    NSDate *datetime1 = [dateFormater dateFromString:string1];
    
    NSDate *datetime2 = [dateFormater dateFromString:string2];
    
    double dateInterval = [datetime2 timeIntervalSinceDate:datetime1] / (60*60*24);
    
    NSMutableString *resultDateDiff = [NSString stringWithFormat:@"DateDiff = %.0f Days",dateInterval];
    lblDateDiff.text = resultDateDiff;
}

// TimeDiff
- (void)TimeDiff
{
    NSString *string1 = @"2012-12-09 15:45:00";
    NSString *string2 = @"2012-12-09 15:50:00";
    
    // Date Format
    NSDateFormatter *dateFormater = [[NSDateFormatter alloc] init];
    [dateFormater setTimeZone:[NSTimeZone timeZoneWithName:@"Asia/Bangkok"]];
    [dateFormater setDateFormat:@"YYYY-MM-dd HH:mm:ss"];
    
    NSDate *datetime1 = [dateFormater dateFromString:string1];
    
    NSDate *datetime2 = [dateFormater dateFromString:string2];
    
    double timeInterval = [datetime2 timeIntervalSinceDate:datetime1] / (60);
    
    NSMutableString *resultTimeDiff = [NSString stringWithFormat:@"TimeDiff = %.2f Minutes",timeInterval];
    lblTimeDiff.text = resultTimeDiff;
}

// Convert Date
- (void)ConvertDate
{
    
    NSDateFormatter *dateFormater = [[NSDateFormatter alloc] init];
    [dateFormater setDateFormat:@"YYYY-MM-dd HH:mm:ss"];
    
    // NSString to NSDate
    NSString *dateString = @"2012-12-09 15:45:00";
    NSDate *dateFromString = [[NSDate alloc] init];
    dateFromString = [dateFormater dateFromString:dateString];
    
    // NSDate to NSString
    NSString *strDate = [dateFormater stringFromDate:[NSDate date]];
    lblConvertDate.text = strDate;
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

- (void)dealloc {
    [lblDateDiff release];
    [lblTimeDiff release];
    [lblConvertDate release];
    [super dealloc];
}
@end