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Julian Date Calculator in AVVSO is Buggy

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Julian Date Calculator in AVVSO is Buggy

Dear all,

I don't know if this forum is the correct place to post this warning. Anyway the Julian date calculator available in the AAVSO site is buggy.


For example 1st January 1 12:00:00 UT is computed as 1721426.00000, the correct answer should be 1721424.00000 according to

Naval Oceanography Portal .mil/USNO/astronomical-applications/data-services/cal-to-jd-conv


Institute de Mecanique Celeste


Moreover 1st Jan -4712 (astronomical year) 12:00:00 is computed as 39.00000 instead of 0 (zero).

For dates useful for observation reporting (i.e. close to 2011) it seems working but the AVVSO calculator is offered as a generic tool and should (IMHO) work for every date.

Hope this helps




Julian Date Calculator in AVVSO is Buggy


After testing, it seems working for Gregorian Date Calendar (from 15th October 1582 onwards) only. For date before 4th October 1582 the AVVSO calculator is not using the Julian Calendar (365.25 days a year).

It seems that the algorithm used by Paris Obesrvatory is

a = (14-month)/12
y = year+4800-a
m = month + 12*a - 3

For a date in the Gregorian calendar:
JD = day + (153*m+2)/5 + y*365 + y/4 - y/100 + y/400 - 32045

For a date in the Julian calendar:
JD = day + (153*m+2)/5 + y*365 + y/4 - 32083

AVVSO seems using just the Gregorian Calendar option.


RWG's picture
AAVSO JD calculator does actually just JD <-> gregCalendar


The page:
   AAVSO Julian Date (JD) Calculator and Calendars
  "Due to the fact that various calendar systems have been in use at different times
   and places around the world, calendar dates prior to October 15, 1582 may be
   translated differently to Julian Date and vice versa. This application assumes use
   of the Gregorian calendar and is most accurate for dates on or after October 15,

This comment (even if it doesn't say it explicitly) states that its working just correctly
onward of October 15, 1582. The formulations are fuzzy and IMO wrong regarding
the Julian and Gregorian Calender to Julian Date conversions as these mapping are
uniquely defined and their mappings are bijective in the mathmatical sense.

It would be better to explicitly state that its actually "just" a JD<->GregCalendar
calculator and that the rsults are just correct for Gregorian calendar dates onwards
of October 15, 1582 and its JD representation.


BSJ's picture
Thank you for your comments

Thank you for your comments Max and Wolfgang.

We have modified the text under the JD calculator so hopefully, it is more clear now.

Best Regards,

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