A simple modal date panel using undocumented methods in Mac OS X Tiger.

datepanel.m

#import <Cocoa/Cocoa.h>

// (C)2018 Cameron Kaiser. BSD license.
// gcc -o datepanel datepanel.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

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

@interface NSPopUpDatePicker : NSAlert {
	NSDatePicker *_picker;
}

- (void)dealloc;
- (NSPopUpDatePicker *)datePicker:(NSString *)message
	defaultButton:(NSString *)defaultButton
	alternateButton:(NSString *)alternateButton
	otherButton:(NSString *)otherButton
	informativeTextWithFormat:(NSString *)format;
- (NSDate *)date;
- (void)setDate:(NSDate *)date;
@end

@interface NSPopUpDatePicker(Private)
- (void)_ensureDatePicker;
- (void)_addDatePickerToAlert;
@end

@implementation NSPopUpDatePicker

- (void)dealloc
{
//	NSLog(@"dealloc");
	[_picker release];
	[super dealloc];
}

- (NSPopUpDatePicker *)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 (NSPopUpDatePicker *)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 _addDatePickerToAlert];
	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 _addDatePickerToAlert];
	return rv;
}

- (NSDate *)date
{
	[self _ensureDatePicker];
	return [_picker dateValue];
}

- (void)setDate:(NSDate *)date
{
	[self _ensureDatePicker];
	[_picker setDateValue:date];
}
@end

@implementation NSPopUpDatePicker(Private)
- (void)_ensureDatePicker
{
	if (!_picker) {
//		NSLog(@"picker init");
		_picker = [[NSDatePicker alloc] initWithFrame:NSMakeRect(10,10,295,154)];
		[_picker setDatePickerStyle:NSClockAndCalendarDatePickerStyle];
		[_picker setDatePickerElements:NSYearMonthDayDatePickerElementFlag];
	}
}
- (void)_addDatePickerToAlert
{
	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 _ensureDatePicker];
	
	// Find the main text field.
	while (subview = [en nextObject]) {
		if ([subview isKindOfClass:[NSTextField class]]) {
			if (++count == 2)
				messageText = (NSTextField *)subview;
		}
	}
	if (messageText) {
		[content addSubview:_picker];
		[_picker sizeToFit];
		
		// Expand the alert window.
		NSRect windowFrame = [window frame];
		NSRect pickerFrame = [_picker frame];
		
		windowFrame.size.height += pickerFrame.size.height + padding;
		[window setFrame:windowFrame display:YES];
		
		// Insert the picker below the main text field.
		pickerFrame.origin.y = [messageText frame].origin.y -
			pickerFrame.size.height - padding;
		pickerFrame.origin.x = [messageText frame].origin.x;
		
		[_picker setFrame:pickerFrame];
		//NSLog(@"Picker installed");
	} else
		NSLog(@"Couldn't find message text, did not add picker");
}
@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;
{
	NSPopUpDatePicker *alert = [NSPopUpDatePicker
		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;
}