计算时间间隔天数

+ (NSInteger)calculateDateInterval:(NSString *)timestamp
{
    NSCalendar *gregorian = [[NSCalendar alloc]
                             initWithCalendarIdentifier:NSGregorianCalendar];
    [gregorian setFirstWeekday:2];
    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
    [dateFormatter setDateFormat:@"yyyy-MM-dd"];
    
    NSDate *fromDate;
    NSDate *toDate;
    [gregorian rangeOfUnit:NSDayCalendarUnit startDate:&fromDate interval:NULL forDate:[dateFormatter dateFromString:timestamp]];
    [gregorian rangeOfUnit:NSDayCalendarUnit startDate:&toDate interval:NULL forDate:[NSDate date]];
    NSDateComponents *dayComponents = [gregorian components:NSDayCalendarUnit fromDate:fromDate toDate:toDate options:0];
    
    return dayComponents.day;
}