gistfile1.m

#import <Foundation/Foundation.h>
#define SECOND_INTERVAL 1
#define MINUTE_INTERVAL SECOND_INTERVAL * 60
#define HOUR_INTERVAL MINUTE_INTERVAL * 60
#define DAY_INTERVAL HOUR_INTERVAL * 24
#define WEEK_INTERVAL DAY_INTERVAL * 7

int main(int argc, char *argv[]) {
	NSAutoreleasePool *p = [[NSAutoreleasePool alloc] init];

	NSDate *now = [NSDate date];
	NSInteger time = (NSInteger)[now timeIntervalSinceReferenceDate];
	
	NSInteger mod = time % DAY_INTERVAL;
	NSInteger expectedValue = (NSInteger)[now timeIntervalSinceReferenceDate];
	
	while (expectedValue - DAY_INTERVAL >= 0)
		expectedValue -= DAY_INTERVAL;

	NSLog(@"Day: %d", DAY_INTERVAL);
	NSLog(@"Time: %d", time);
	NSLog(@"Modded value: %d", mod);
	NSLog(@"Loopy value: %d", expectedValue);
	NSLog(@"Is this right? %@", expectedValue == mod ? @"Why yes, yes it is..." : @"Nope, magic C things are breaking!");
	[p release];
}

output:
2012-08-14 01:15:04.550 Untitled 2[30690:707] Day: 86400
2012-08-14 01:15:04.554 Untitled 2[30690:707] Time: 366617704
2012-08-14 01:15:04.554 Untitled 2[30690:707] Modded value: 0
2012-08-14 01:15:04.555 Untitled 2[30690:707] Loopy value: 22504
2012-08-14 01:15:04.556 Untitled 2[30690:707] Is this right? Nope, magic C things are breaking!

Days are not always 86400 seconds long. This type of calculation is almost always the Wrong Thing. Use -[NSCalendar components:fromDate:toDate:options:] instead.

Yep, good call. Defines were easier for terrible prototyping :)