2つの日付の差を日数で出す
int dayCount = [self getDaysCountByTwoDateString:@"2014-04-01" endDateString:@"2014-04-04"];
-(int)getDaysCountByTwoDateString:(NSString*)startDateString endDateString:(NSString*)endDateString
{
    NSDateFormatter *inputDateFormatter = [[NSDateFormatter alloc] init];
    [inputDateFormatter setDateFormat:@"yyyy-MM-dd"];
    
    NSDate *startInputDate = [inputDateFormatter dateFromString:startDateString];
    NSDate *endInputDate = [inputDateFormatter dateFromString:endDateString];
    
    float tmp= [endInputDate timeIntervalSinceDate:startInputDate];
    int day=(int)( tmp / (3600.0*24.0) );
    return day;
}