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 — Converte um calendário (suportado) para o calendário "Julian Day Count"
   cal_to_jd() calcula uma data em formato "Julian Day Count"
   à partir de um calendar suportado especificado.
   Os calendar suportados são:
   CAL_GREGORIAN,
   CAL_JULIAN,
   CAL_JEWISH e
   CAL_FRENCH.
  
calendar
       Calendário para converter, um dos
       CAL_GREGORIAN,
       CAL_JULIAN,
       CAL_JEWISH ou
       CAL_FRENCH.
      
month
       O mês como um número, o intervalo válido depende
       do calendar
      
day
       O dia como um número, o intervalo válido depende
       do calendar
      
year
       O ano como um número, o intervalo válido depende
       do calendar
      
Um número de data 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.