calendar:date_to_gregorian_days/1

返回从公元零年到所给出的时间之间的天数

用法：

date_to_gregorian_days(Date) -> Days

内部实现：

-spec date_to_gregorian_days(Year, Month, Day) -> Days when
      Year :: year(),
      Month :: month(),
      Day :: day(),
      Days :: non_neg_integer().
date_to_gregorian_days(Year, Month, Day) when is_integer(Day), Day > 0 ->
    Last = last_day_of_the_month(Year, Month),
    if
	Day =< Last ->
	    dy(Year) + dm(Month) + df(Year, Month) + Day - 1
    end.

-spec date_to_gregorian_days(Date) -> Days when
      Date :: date(),
      Days :: non_neg_integer().
date_to_gregorian_days({Year, Month, Day}) ->
    date_to_gregorian_days(Year, Month, Day).

获取从公元零年到所给出的时间 Date 之间的天数：

calendar:date_to_gregorian_days({1970, 1, 1}).

calendar:date_to_gregorian_days({2013, 9, 18}).