日期与累积日之间的换算。

例如，1月1日是第一天，非闰年的12月31日是365天，而闰年的话12月31日是366天。-date with the cumulative days between the conversion. For example, on January 1 is the first day of the non-leap year on December 31 is 365 days, and the leap year on December 31 is 366 days.