上篇文章《C#中时间相关知识点小结》介绍了各种日期操作的小知识点，有网友问有没有实现农历和公历的转换源码，这个可以明确的告诉他，网上一搜索一大堆，我们不能只看代码，得研究它的算法。快过年了，大家也可以通过文章中的计算方法编写过年日历。这篇文章介绍一下公历和农历的转换算法，并附上源码。
一、公历和农历的来历
公历称为公元，即公历纪年法，是一种源自于西方社会的纪年方法，原称基督纪元，又称西历或西元，是由意大利医生兼哲学家Aloysius Lilius对儒略历加以改革而制成的一种历法——格里历。1582年，时任罗马教皇的格列高利十三世予以批准颁行。.
农历，是中国现行的近现代历法，属于阴阳合历，也就是阴历和阳历的合历，是根据月相的变化周期，每一次月相朔望变化为一个月，参考太阳回归年为一年的长度，并加入二十四节气与设置闰月以使平均历年与回归年相适应。农历融合阴历与阳历形成为一种阴阳合历历法，因使用“夏正”，古时称为夏历。
需要注意的是农历并不是古代的历法，而是跟古代历法融合的现代立法，农历是1970年以后使用新的历法，其年份分为平年和闰年，平年为十二个月，闰年为十三个月，月份分为大月和小月，大月三十天，小月二十九天，其平均历月等于一个朔望月。①
二、转换算法
公历和农历转换没有具体的方法，据说是根据天文台根据月亮周期测算出来的，但是网上还是有很多算法，小编也实验过很多网上的算法，还是github的算法靠谱。下面是github上面的C#计算农历的方法②，小编增加了一个日期直接转换的方法，已经经过本人验证，大家可以参考一下。
// 类internal class Lunar{private static int LunarCalendarDay;private static readonly int[] LunarCalendarTable ={0x04AE53,0x0A5748,0x5526BD,0x0D2650,0x0D9544,0x46AAB9,0x056A4D,0x09AD42,0x24AEB6,0x04AE4A,/*1901-1910*/0x6A4DBE,0x0A4D52,0x0D2546,0x5D52BA,0x0B544E,0x0D6A43,0x296D37,0x095B4B,0x749BC1,0x049754,/*1911-1920*/0x0A4B48,0x5B25BC,0x06A550,0x06D445,0x4ADAB8,0x02B64D,0x095742,0x2497B7,0x04974A,0x664B3E,/*1921-1930*/0x0D4A51,0x0EA546,0x56D4BA,0x05AD4E,0x02B644,0x393738,0x092E4B,0x7C96BF,0x0C9553,0x0D4A48,/*1931-1940*/0x6DA53B,0x0B554F,0x056A45,0x4AADB9,0x025D4D,0x092D42,0x2C95B6,0x0A954A,0x7B4ABD,0x06CA51,/*1941-1950*/0x0B5546,0x555ABB,0x04DA4E,0x0A5B43,0x352BB8,0x052B4C,0x8A953F,0x0E9552,0x06AA48,0x6AD53C,/*1951-1960*/0x0AB54F,0x04B645,0x4A5739,0x0A574D,0x052642,0x3E9335,0x0D9549,0x75AABE,0x056A51,0x096D46,/*1961-1970*/0x54AEBB,0x04AD4F,0x0A4D43,0x4D26B7,0x0D254B,0x8D52BF,0x0B5452,0x0B6A47,0x696D3C,0x095B50,/*1971-1980*/0x049B45,0x4A4BB9,0x0A4B4D,0xAB25C2,0x06A554,0x06D449,0x6ADA3D,0x0AB651,0x093746,0x5497BB,/*1981-1990*/0x04974F,0x064B44,0x36A537,0x0EA54A,0x86B2BF,0x05AC53,0x0AB647,0x5936BC,0x092E50,0x0C9645,/*1991-2000*/0x4D4AB8,0x0D4A4C,0x0DA541,0x25AAB6,0x056A49,0x7AADBD,0x025D52,0x092D47,0x5C95BA,0x0A954E,/*2001-2010*/0x0B4A43,0x4B5537,0x0AD54A,0x955ABF,0x04BA53,0x0A5B48,0x652BBC,0x052B50,0x0A9345,0x474AB9,/*2011-2020*/0x06AA4C,0x0AD541,0x24DAB6,0x04B64A,0x69573D,0x0A4E51,0x0D2646,0x5E933A,0x0D534D,0x05AA43,/*2021-2030*/0x36B537,0x096D4B,0xB4AEBF,0x04AD53,0x0A4D48,0x6D25BC,0x0D254F,0x0D5244,0x5DAA38,0x0B5A4C,/*2031-2040*/0x056D41,0x24ADB6,0x049B4A,0x7A4BBE,0x0A4B51,0x0AA546,0x5B52BA,0x06D24E,0x0ADA42,0x355B37,/*2041-2050*/0x09374B,0x8497C1,0x049753,0x064B48,0x66A53C,0x0EA54F,0x06B244,0x4AB638,0x0AAE4C,0x092E42,/*2051-2060*/0x3C9735,0x0C9649,0x7D4ABD,0x0D4A51,0x0DA545,0x55AABA,0x056A4E,0x0A6D43,0x452EB7,0x052D4B,/*2061-2070*/0x8A95BF,0x0A9553,0x0B4A47,0x6B553B,0x0AD54F,0x055A45,0x4A5D38,0x0A5B4C,0x052B42,0x3A93B6,/*2071-2080*/0x069349,0x7729BD,0x06AA51,0x0AD546,0x54DABA,0x04B64E,0x0A5743,0x452738,0x0D264A,0x8E933E,/*2081-2090*/0x0D5252,0x0DAA47,0x66B53B,0x056D4F,0x04AE45,0x4A4EB9,0x0A4D4C,0x0D1541,0x2D92B5 /*2091-2099*/};protected static int[] MonthAdd = { 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334 };protected static int LunarCalendar(int year, int month, int day){//StaticDayCount记录大小月的天数 29 或30//index 记录从哪个月开始来计算。//flag 是用来对闰月的特殊处理。int Spring_NY, Sun_NY, StaticDayCount;int index, flag;if (((LunarCalendarTable[year - 1901] & 0x0060) >> 5) == 1)Spring_NY = (LunarCalendarTable[year - 1901] & 0x001F) - 1;elseSpring_NY = (LunarCalendarTable[year - 1901] & 0x001F) + 30;Sun_NY = MonthAdd[month - 1] + day - 1;if (((year % 4 == 0)) && (month > 2))Sun_NY++;//判断日期是否再春节后if (Sun_NY >= Spring_NY){Sun_NY -= Spring_NY;month = 1;index = 1;flag = 0;if ((LunarCalendarTable[year - 1901] & (0x80000 >> (index - 1))) == 0)StaticDayCount = 29;elseStaticDayCount = 30;while (Sun_NY >= StaticDayCount){Sun_NY -= StaticDayCount;index++;if (month == ((LunarCalendarTable[year - 1901] & 0xF00000) >> 20)){flag = ~flag;if (flag == 0)month++;}elsemonth++;if ((LunarCalendarTable[year - 1901] & (0x80000 >> (index - 1))) == 0)StaticDayCount = 29;elseStaticDayCount = 30;}day = Sun_NY + 1;}else{Spring_NY -= Sun_NY;year--;month = 12;if (((LunarCalendarTable[year - 1901] & 0xF00000) >> 20) == 0)index = 12;elseindex = 13;flag = 0;if ((LunarCalendarTable[year - 1901] & (0x80000 >> (index - 1))) == 0)StaticDayCount = 29;elseStaticDayCount = 30;while (Spring_NY > StaticDayCount){Spring_NY -= StaticDayCount;index--;if (flag == 0)month--;if (month == ((LunarCalendarTable[year - 1901] & 0xF00000) >> 20))flag = ~flag;if ((LunarCalendarTable[year - 1901] & (0x80000 >> (index - 1))) == 0)StaticDayCount = 29;elseStaticDayCount = 30;}day = StaticDayCount - Spring_NY + 1;}LunarCalendarDay |= day;LunarCalendarDay |= (month << 6);if (month == ((LunarCalendarTable[year - 1901] & 0xF00000) >> 20))return 1;elsereturn 0;}public static bool IsLeap(int year, int month, int day){return Convert.ToBoolean(LunarCalendar(year, month, day));}public static bool IsLeap(){return Convert.ToBoolean(LunarCalendar(DateTime.Now.Year, DateTime.Now.Month, DateTime.Now.Day));}public int GetData(){return LunarCalendarDay;}public enum Lang{English = 0,Chinese = 2052,French = 1036,Japanese = 1041,Chinese_Traditional = 1028,Chinese_Simplified = 2052,Chinese_HongKong = 3076,Chinese_Taiwan = 1028,Chinese_PRC = 2052}public static string GetString(Lang lang, int year, int month, int day){LunarDate ld = new LunarDate();ld = GetLunarDate(year, month, day);if (lang == Lang.English)return Convert.ToString(ld.Month) + "-" + Convert.ToString(ld.Day);else if (lang == Lang.Chinese){string[] MonthTable = { "*", "正月", "二月", "三月", "四月", "五月", "六月", "七月", "八月", "九月", "十月", "冬月", "腊月" };string[] DayTable = { "*", "初一", "初二", "初三", "初四", "初五", "初六", "初七", "初八", "初九", "初十", "十一", "十二", "十三", "十四", "十五", "十六", "十七", "十八", "十九", "二十", "廿一", "廿二", "廿三", "廿四", "廿五", "廿六", "廿七", "廿八", "廿九", "三十" };return MonthTable[ld.Month] + DayTable[ld.Day];}else if (lang == Lang.Japanese){string[] MonthTable = { "*", " 1月", " 2月", " 3月", " 4月", " 5月", " 6月", " 7月", " 8月", " 9月", "10月", "11月", "12月" };string result = MonthTable[ld.Month] + Convert.ToString(ld.Day) + "日";if (ld.Day <= 10)result += "目";return result;}else if (lang == Lang.French)return Convert.ToString(ld.Day) + "/" + Convert.ToString(ld.Month);else if ((lang == Lang.Chinese_Traditional) || (lang == Lang.Chinese_HongKong) || (lang == Lang.Chinese_Taiwan)){string[] MonthTable = { "*", "正月", "二月", "三月", "四月", "五月", "六月", "七月", "八月", "九月", "十月", "冬月", "臘月" };string[] DayTable = { "*", "初一", "初二", "初三", "初四", "初五", "初六", "初七", "初八", "初九", "初十", "十一", "十二", "十三", "十四", "十五", "十六", "十七", "十八", "十九", "二十", "廿一", "廿二", "廿三", "廿四", "廿五", "廿六", "廿七", "廿八", "廿九", "三十" };return MonthTable[ld.Month] + DayTable[ld.Day];}elsereturn Convert.ToString(ld.Month) + "-" + Convert.ToString(ld.Day);}public struct LunarDate{public int Month;public int Day;public int Year;public bool IsLeap;}public static LunarDate GetLunarDate(int year, int month, int day){LunarDate ld = new LunarDate();LunarCalendarDay = 0;if (year < 1901)throw new ArgumentOutOfRangeException(nameof(year));if (year > 2099)throw new ArgumentOutOfRangeException(nameof(year));if ((month <= 0) || (month >= 12))throw new ArgumentOutOfRangeException(nameof(month));if ((month == 2) && ((day <= 0) || (day > 29)))throw new ArgumentOutOfRangeException(nameof(day));else if (((month == 1) || (month == 3) || (month == 5) || (month == 7) || (month == 8) || (month == 10) || (month == 12)) && ((day <= 0) || (day > 31)))throw new ArgumentOutOfRangeException(nameof(day));else if (((month == 4) || (month == 6) || (month == 9) || (month == 11)) && ((day <= 0) || (day > 30)))throw new ArgumentOutOfRangeException(nameof(day));if (LunarCalendar(year, month, day) != 0)ld.IsLeap = true;ld.Month = (LunarCalendarDay & 0x3C0) >> 6;ld.Day = LunarCalendarDay & 0x3F;ld.Year = ld.Month > month ? year - 1: year;return ld;}public static LunarDate GetLunarDate(){return GetLunarDate(DateTime.Now.Year, DateTime.Now.Month, DateTime.Now.Day);}public DateTime GetLunarDateTime(DateTime dateTime){DateTime dt = new DateTime();var d= GetLunarDate(dateTime.Year, dateTime.Month, dateTime.Day);var rdateTime =d.Year+"-"+ d.Month + "-" + d.Day;return DateTime.Parse(rdateTime);}}//调用Lunar lunar = new Lunar();Lunar.LunarDate ld = new Lunar.LunarDate();ld = Lunar.GetLunarDate();Console.Write(ld.Month);Console.Write('.');Console.WriteLine(ld.Day);//当天的阴历var rel = lunar.GetLunarDateTime(DateTime.Parse("2023-01-23"));Console.Write(rel);//2023-01-02
三、通过官方类库
微软官方也提供了日期转换的类库，使用System.Globalization类库，这个是比较通用的方法，网上很多这种例子，可以实现公历农历互相转换，如果感兴趣可以尝试一下，这里就不列出使用方法了。
参考① ：百度百科
参考② ：https://github.com/LS-KR/LunarMonthCalender
