NSFileManager *fileManager = [NSFileManager defaultManager];
NSDate *now = [NSDate date];
NSDictionary *timestampAttr = [NSDictionary dictionaryWithObjectsAndKeys:
                                      now, NSFileCreationDate, now, NSFileModificationDate, nil];
BOOL ret = [fileManager setAttributes:timestampAttr ofItemAtPath:path error:&err];

#import <Cocoa/Cocoa.h>

int main(void)
{
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
    NSFileManager *m = [NSFileManager defaultManager];
    NSString *path = ...;
    [m setAttributes:[NSDictionary dictionaryWithObjectsAndKeys:[NSDate date],NSFileCreationDate,nil] ofItemAtPath:path error:nil];
    [pool drain];
    return 0;
}

NSFileManager *fileManager = [NSFileManager defaultManager];
NSDate *now = [NSDate date];
NSDictionary *creationDateAttr = [NSDictionary dictionaryWithObjectsAndKeys: now, NSFileCreationDate, nil];
NSDictionary *modificationDateAttr = [NSDictionary dictionaryWithObjectsAndKeys: now, NSFileModificationDate, nil];
[fileManager setAttributes:modificationDateAttr ofItemAtPath:path error:&err];
[fileManager setAttributes:creationDateAttr ofItemAtPath:path error:&err];