gistfile1.txt

//
//  AppDelegate.m
//  MachHack2
//
//  Created by Chris Hamons on 3/20/15.
//  Copyright (c) 2015 Chris Hamons. All rights reserved.
//

#import "AppDelegate.h"
@import AppKit;
@import QuickLook;
@import Quartz;
@import GLKit;
@import SecurityInterface;

@interface AppDelegate ()

@property (weak) IBOutlet NSWindow *window;
@end

@implementation AppDelegate

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {
    NSDatePicker * p = [NSDatePicker new];
    p.frame = NSMakeRect(0, 0, 200, 200);
    p.datePickerElements = NSYearMonthDatePickerElementFlag;
    p.datePickerMode = NSSingleDateMode;
    p.timeZone = NSTimeZoneNameStyleStandard;
    p.datePickerStyle = NSClockAndCalendarDatePickerStyle;
    p.bordered = false;
    p.dateValue = [NSDate date];
    p.delegate = self;
    [self.window.contentView addSubview:p];

}

- (void)applicationWillTerminate:(NSNotification *)aNotification {
    // Insert code here to tear down your application
}

- (void)datePickerCell:(NSDatePickerCell *)aDatePickerCell validateProposedDateValue:(NSDate **)proposedDateValue timeInterval:(NSTimeInterval *)proposedTimeInterval
{
    NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
    [formatter setDateFormat:@"YYYY-MM-dd"];
    NSLog(@"Date is %@",[formatter stringFromDate:*proposedDateValue]);
}

@end

Add this to your h file

@interface AppDelegate : NSObject <NSApplicationDelegate, NSDatePickerCellDelegate>