将儒略历日期转换为公历日期时年份出错

Question

我需要将儒略历（从API提取）转换为公历。我点击了以下链接：Converting a Gregorian date to Julian Day Count in Objective C。但是，年份组件会同时引起两种混淆（1）：

NSInteger julianDays = 1580198400;
NSUInteger julianDayFor01012000 = 2451545;
NSCalendar *cal = [[NSCalendar alloc]           initWithCalendarIdentifier:NSCalendarIdentifierGregorian];
[cal setTimeZone:[NSTimeZone timeZoneForSecondsFromGMT:0]];
NSDateComponents *comp = [[NSDateComponents alloc] init];
comp.year = 2000;
comp.month = 1;
comp.day = 1;
NSDate *ref = [cal dateFromComponents:comp];
NSDateComponents *diff = [[NSDateComponents alloc] init];
diff.day = julianDays - julianDayFor01012000;
NSDate *date = [cal dateByAddingComponents:diff toDate:ref options:0];
comp = [cal components:NSCalendarUnitDay|NSCalendarUnitMonth|NSCalendarUnitYear fromDate:date];

输出：日历年：4321724月份：3 Le月：否天：3

和（2）：

NSInteger julianDays = 1580198400;
NSDateFormatter *fmt = [[NSDateFormatter alloc] init];
[fmt setDateFormat:@"g"];
NSDate *date = [fmt dateFromString:[@(julianDays) stringValue]];
NSCalendar *cal = [[NSCalendar alloc] initWithCalendarIdentifier:NSCalendarIdentifierGregorian];
NSDateComponents *comp = [cal components:NSCalendarUnitDay|NSCalendarUnitMonth|NSCalendarUnitYear fromDate:date];

输出：日历年：4321724月份：3 Le月：否天：3

为什么今年是4327724？我需要进一步解释吗？任何帮助将不胜感激。