Parallel Julian / Gregorian Calendar Calculator
For Any Given Date On Either Calendar
PHP Program by Jay Tanner
Calendar Date:   ±Y  M  D   (Neg Year = BC)
Calendar Input Mode:    Julian              Gregorian 
J   2023     March
SunMonTueWedThuFriSat
    1 2 3 4 5
6 7 8 9 10 11 12
13 14 15 16 17 18 19
20 21 22 23 24 25 26
27 28 29 30 31    
G   2023     March
SunMonTueWedThuFriSat
      1 2 3 4
5 6 7 8 9 10 11
12 13 14 15 16 17 18
19 20 21 22 23 24 25
26 27 28 29 30 31   

 DATE = March 27, 2023 = Monday on the Gregorian Calendar  

 JD Number = 2460031 on Gregorian calendar  
           = JD (Julian Date) at 12:00:00 on date  

 Julian Calendar Date = Gregorian Calendar Date - 13 days 

 Note that the corresponding days of the week are the same on both 
 calendars no matter the number of days difference between them.