An expansion of `datepanel.m` that displays synchronized date pickers in a modal dialogue using undocumented methods in Mac OS X Tiger.

doubledate.m

#import <Cocoa/Cocoa.h>

// (C)2018 Cameron Kaiser. BSD license.
// gcc -o doubledate doubledate.m -framework Cocoa

// 10.4 does not have an accessory view method for NSAlert, but we can
// simulate it with these undocumented methods.
@interface NSAlert(WithCustomStyle)
- (void)prepare;
- (id)buildAlertStyle:(int)fp8 title:(id)fp12 formattedMsg:(id)fp16 first:(id)fp20 second:(id)fp24 third:(id)fp28 oldStyle:(BOOL)fp32;
- (id)buildAlertStyle:(int)fp8 title:(id)fp12 message:(id)fp16 first:(id)fp20 second:(id)fp24 third:(id)fp28 oldStyle:(BOOL)fp32 args:(char *)fp36;
@end

@class NSDoubleDatePicker;

@interface NSDoubleDateDelegate : NSObject {
	NSDoubleDatePicker *_parentAlert;
	NSDatePicker *_source;
}

- (void)datePickerCell:(NSDatePickerCell *)aDatePickerCell
	validateProposedDateValue:(NSDate **)proposedDateValue
	timeInterval:(NSTimeInterval *)proposedTimeInterval;
- (void)setParentAlert:(NSDoubleDatePicker *)parentAlert
	withSource:(NSDatePicker *)source;
@end

@implementation NSDoubleDateDelegate
- (void)datePickerCell:(NSDatePickerCell *)aDatePickerCell
	validateProposedDateValue:(NSDate **)proposedDateValue
	timeInterval:(NSTimeInterval *)proposedTimeInterval
{
	NSLog(@"validate");
	[_parentAlert onSwitchControl:_source newDate:proposedDateValue];
}

- (void)setParentAlert:(NSDoubleDatePicker *)parentAlert
	withSource:(NSDatePicker *)source
{
	_parentAlert = parentAlert;
	_source = source;
}
@end         

////// NSDoubleDatePicker
////// based on NSAlertCheckbox, http://cocoadev.github.io/NSAlertCheckbox/

@interface NSDoubleDatePicker : NSAlert {
	NSDatePicker *_pickertop;
	NSDatePicker *_pickerbottom;
	NSDoubleDateDelegate *_topdelegate;
	NSDoubleDateDelegate *_bottomdelegate;
}

- (void)dealloc;
- (NSDoubleDatePicker *)datePicker:(NSString *)message
	defaultButton:(NSString *)defaultButton
	alternateButton:(NSString *)alternateButton
	otherButton:(NSString *)otherButton
	informativeTextWithFormat:(NSString *)format;
- (void)onSwitchControl:(NSDatePicker *)which newDate:(NSDate **)newDate;
- (NSDate *)date;
- (void)setDate:(NSDate *)date;
@end

@interface NSDoubleDatePicker(Private)
- (void)_ensureDatePickers;
- (void)_addDatePickersToAlert;
@end

@implementation NSDoubleDatePicker

- (void)dealloc
{
//	NSLog(@"dealloc");
	[_pickertop release];
	[_pickerbottom release];
	[_topdelegate release];
	[_bottomdelegate release];
	[super dealloc];
}

- (NSDoubleDatePicker *)datePicker:(NSString *)message
	defaultButton:(NSString *)defaultButton
	alternateButton:(NSString *)alternateButton
	otherButton:(NSString *)otherButton
	informativeTextWithFormat:(NSString *)format
{
	NSAlert *alert = [super alertWithMessageText:message
	                               defaultButton:defaultButton
	                             alternateButton:alternateButton
	                                 otherButton:otherButton
	                   informativeTextWithFormat:format];
	return (NSDoubleDatePicker *)alert;
}

- (id)buildAlertStyle:(int)fp8
	title:(id)fp12
	formattedMsg:(id)fp16
	first:(id)fp20
	second:(id)fp24
	third:(id)fp28
	oldStyle:(BOOL)fp32
{
//	NSLog(@"buildAlertStyle 1");
	id rv = [super buildAlertStyle:fp8
				title:fp12
				formattedMsg:fp16
				first:fp20
				second:fp24
				third:fp28
				oldStyle:fp32];
	[self _addDatePickersToAlert];
	return rv;
}

- (id)buildAlertStyle:(int)fp8
	title:(id)fp12
	message:(id)fp16
	first:(id)fp20
	second:(id)fp24
	third:(id)fp28
	oldStyle:(BOOL)fp32
	args:(char *)fp36
{
//	NSLog(@"buildAlertStyle 2");
	id rv = [super buildAlertStyle:fp8
				title:fp12
				message:fp16
				first:fp20
				second:fp24
				third:fp28
				oldStyle:fp32
				args:fp36];
	[self _addDatePickersToAlert];
	return rv;
}

- (void)onSwitchControl:(NSDatePicker *)which newDate:(NSDate **)newDate
{
	// Halt the delegate on the one we're setting first.
	if (which == _pickertop) {
		NSLog(@"control event: top");
		[_pickerbottom setDelegate:nil];
		[_pickerbottom setDateValue:*newDate];
		[_pickerbottom setDelegate:_bottomdelegate];
	} else if (which == _pickerbottom) {
		NSLog(@"control event: bottom");
		[_pickertop setDelegate:nil];
		[_pickertop setDateValue:*newDate];
		[_pickertop setDelegate:_topdelegate];
	} else
		NSLog(@"wtf");
}

- (NSDate *)date
{
	[self _ensureDatePickers];
	return [_pickertop dateValue];
}

- (void)setDate:(NSDate *)date
{
	[self _ensureDatePickers];
	[_pickertop setDateValue:date];
	[_pickerbottom setDateValue:date];
}
@end

@implementation NSDoubleDatePicker(Private)
- (void)_ensureDatePickers
{
	if (!_pickertop) {
//		NSLog(@"picker init");
		_pickertop = [[NSDatePicker alloc] initWithFrame:NSMakeRect(10,10,295,154)];
		[_pickertop setDatePickerStyle:NSClockAndCalendarDatePickerStyle];
		[_pickertop setDatePickerElements:NSYearMonthDayDatePickerElementFlag];
		
		_topdelegate = [[NSDoubleDateDelegate alloc] init];
		[_topdelegate setParentAlert:self withSource:_pickertop];
		[_pickertop setDelegate:_topdelegate];

		_pickerbottom = [[NSDatePicker alloc] initWithFrame:NSMakeRect(10,10,295,154)];
		[_pickerbottom setDatePickerStyle:NSTextFieldAndStepperDatePickerStyle];
		[_pickerbottom setDatePickerElements:NSYearMonthDayDatePickerElementFlag];
		
		_bottomdelegate = [[NSDoubleDateDelegate alloc] init];
		[_bottomdelegate setParentAlert:self withSource:_pickerbottom];
		[_pickerbottom setDelegate:_bottomdelegate];
	}
}
- (void)_addDatePickersToAlert
{
	NSWindow *window = [self window];
	NSView *content = [window contentView];
	float padding = 14.0f;
	
	NSArray *subviews = [content subviews];
	NSEnumerator *en = [subviews objectEnumerator];
	NSView *subview = nil;
	NSTextField *messageText = nil;
	int count = 0;
	
	[self _ensureDatePickers];
	
	// Find the main text field.
	while (subview = [en nextObject]) {
		if ([subview isKindOfClass:[NSTextField class]]) {
			if (++count == 2)
				messageText = (NSTextField *)subview;
		}
	}
	if (messageText) {
		[content addSubview:_pickertop];
		[_pickertop sizeToFit];
		[content addSubview:_pickerbottom];
		[_pickerbottom sizeToFit];
		
		// Expand the alert window.
		NSRect windowFrame = [window frame];
		NSRect topPickerFrame = [_pickertop frame];
		NSRect bottomPickerFrame = [_pickerbottom frame];
		
		windowFrame.size.height += topPickerFrame.size.height + padding +
			bottomPickerFrame.size.height + padding;
		[window setFrame:windowFrame display:YES];
		
		// Insert the pickers below the main text field.
		topPickerFrame.origin.y = [messageText frame].origin.y -
			bottomPickerFrame.size.height - padding -
			topPickerFrame.size.height - padding;
		topPickerFrame.origin.x = [messageText frame].origin.x;
		
		bottomPickerFrame.origin.y = topPickerFrame.origin.y +
			topPickerFrame.size.height + padding;
		bottomPickerFrame.origin.x = topPickerFrame.origin.x;
		
		[_pickertop setFrame:topPickerFrame];
		[_pickerbottom setFrame:bottomPickerFrame];
		//[window makeMainWindow];
		[window makeKeyWindow];
		//[window orderFrontRegardless];
		//NSLog(@"Picker installed");
	} else
		NSLog(@"Couldn't find message text, did not add pickers");
}
@end

////// App delegate runner

@interface AppDelegate : NSObject
- (id) init;
- (void) applicationWillFinishLaunching: (NSNotification*) aNotification;
- (void) applicationDidFinishLaunching: (NSNotification*) aNotification;
@end

@implementation AppDelegate
- (id) init
{
	return [super init];
}
- (void) applicationWillFinishLaunching: (NSNotification*) aNotification
{
//	NSLog(@"WillFinish");
}
- (void) applicationDidFinishLaunching: (NSNotification*) aNotification;
{
	NSDoubleDatePicker *alert = [NSDoubleDatePicker
		alertWithMessageText:@"One"
		defaultButton:@"OK"
		alternateButton:@"Cancel"
		otherButton:nil
		informativeTextWithFormat:@"Blah blah"];	
	[alert setDate:[NSDate date]];
	int32_t result = [alert runModal];

	NSDate *date = [alert date];
	NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
	[formatter setFormatterBehavior:NSDateFormatterBehavior10_4];
	[formatter setDateFormat:@"yyyy-MM-dd"];
	NSString *stringFromDate = [formatter stringFromDate:date];

	printf("%d %s\n", result, [stringFromDate UTF8String]);
	[[NSApplication sharedApplication] terminate:nil];
}
@end

int main(int argc, char **argv) {
	NSAutoreleasePool *shutup = [[NSAutoreleasePool alloc] init];
        NSApplication *application = [NSApplication sharedApplication];
        AppDelegate *applicationDelegate = [[AppDelegate alloc] init]; 
        [application setDelegate:applicationDelegate];
        [application run];
        
        return 0;
}