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.(PHP 4 >= 4.1.0, PHP 5, PHP 7, PHP 8)
cal_to_jd — Convertir un calendario soportado a la Fecha Juliana
   cal_to_jd() calcula la Fecha Juliana
   para una fecha en el calendario calendar especificado.
   Los calendarios calendar soportados son
   CAL_GREGORIAN,
   CAL_JULIAN,
   CAL_JEWISH y
   CAL_FRENCH.
  
calendar
       El calendario desde el que se va a hacer la conversión,
       CAL_GREGORIAN,
       CAL_JULIAN,
       CAL_JEWISH o
       CAL_FRENCH.
      
month
       El mes como número, el rango válido depende de calendar
      
day
       El día como número, el rango válido depende de calendar
      
year
       El año como número, el rango válido depende de calendar
      
Un número de día de la Fecha Juliana.
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.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.