NSNumber from NSDate

I'm attempting to get around a date validation that refuses to take anything earlier than tomorrow.
So far I have this:

NSDate *dateY = [NSDate dateWithTimeIntervalSinceNow:-86400];
                // Negative one day, in seconds (-60*60*24)
NSLog(@"%@", [NSDate date]);
    // "yyyy-MM-dd HH:mm:ss Z", accurate assuming Z = +0000
NSLog(@"%@", dateY);
    // "yyyy-MM-dd HH:mm:ss Z", same accuracy (minus one day)

That's great, but dateY is not an NSNumber. I need an NSNumber for the comparison, but I can't find anything that works. (I don't even know how an NSNumber can be 2011-04-14 13:22:29 +0000, anyway...)

I can use NSDateFormatter to convert an NSDate into an NSString, so if it would be possible to take that string and convert it to the required NSNumber (as opposed to directly converting the NSDate to an NSNumber, which I can't seem to find help with either), that would be fine.

- (BOOL)validateDueDate:(id *)ioValue error:(NSError **)outError {
    NSDate *dateY = [NSDate dateWithTimeIntervalSinceNow:-86400];
    NSNumber *tis1970 = [NSNumber numberWithDouble:[dateY timeIntervalSince1970]];
    NSLog(@"NSNumber From Date : %@", tis1970);
    NSLog(@"Date From NSNumber : %@", [NSDate dateWithTimeIntervalSince1970:[tis1970 doubleValue]]);

    // Due dates in the past are not valid
    // Enforced that a due date has to be >= today's date
    if ([*ioValue compare:[NSDate date]] == NSOrderedAscending) {
        if (outError != NULL) {
            NSString *errorStr = [[[NSString alloc] initWithString:@"Due date must be today or later."] autorelease];
            NSDictionary *userInfoDictionary = [NSDictionary dictionaryWithObject:errorStr forKey:@"ErrorString"];
            NSError *error = [[[NSError alloc]
                                initWithDom