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   2025     June
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            
G   2025     June
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           

 DATE = June 16, 2025 = Monday on the Gregorian Calendar  

 JD Number = 2460843 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.