(PHP 4 >= 4.1.0, PHP 5, PHP 7, PHP 8)
cal_to_jd — 从一个支持的历法转变为Julian Day计数。
$calendar
, int $month
, int $day
, int $year
) : int
cal_to_jd()函数从一个给定的历法日期计算出Julian天数，支持的历法有 CAL_GREGORIAN，CAL_JULIAN，CAL_JEWISH和CAL_FRENCH。
calendar
选定的历法，可以是CAL_GREGORIAN，CAL_JULIAN，CAL_JEWISH或CAL_FRENCH中的某一个。
month
数字形式的月份，根据选定的calendar历法来确定范围。
day
数字形式的日期，根据选定的calendar历法来确定范围。
year
数字形式的年份，根据选定的calendar历法来确定范围。
一个Julian天数。
The earliest supported date in the Julian calendar is January 2, 4713 BC. The corresponding Julian day count is 1. PHP treats -4713 as equal to 4713 BC, and does not allow the year 0; this practice is different than the custom of astronomers, who treat -4712 as equal to 4713 BC.
Julian days begin at noon, while the days of the Gregorian and Julian calendars are usually thought of as beginning at midnight. For the Gregorian and Julian calendars, the function behaves as if the conversion were performed at noon.