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2021-04-18 6:42:46 UTC
ISO format (YYYY-MM-DD HH:MM:SS UTC)°
||April 18, 2021
||April 5, 2021 (post ¹)
||Iyyar 6, 5781
ו׳ באייר ה׳תשפ״א
If you wish to view another year, change the date values below click the button, then click the link above.
* Except for the Jewish calendar, years Before Christ (BC) are entered
with a minus sign, such as -500.
** To subtract from the SDN use a minus sign like this: -500 and select SDN
This program is limited to reasonable dates. Valid entries are:
November 25, -4714 to December 31, 10000 in the Gregorian
Tishri 1, 1 to 10 Kislev 13761 in the Jewish calendar.
January 2, -4713 to 19 October 10000 in the Julian calendar.
1 to 5373850 in the serial day number (used by astronomers), also
called Julian day, not to be confused with the Julian calendar.
? This signifies that the calendar had not been in use on the date chosen, yet
reflects the date as if it had been in use. Check out September 2, 1752 in the
Julian calendar and then type 1 in the SDN+ box and click the
SDN+ button and watch the calendar change to Gregorian. The switch from Julian
calendar to the Gregorian calendar in the English speaking world took place on
Julian date Wednesday, September 3, 1752 when 11 days were removed making the
new date Thursday, September 14, 1752. The Julian was created July 1, 45 B.C.
Before that the Roman calendar was used - not shown here. Throughout, the SDN
and Jewish calendars remained constant as far as history can tell.
? With the exception of first visiting this page and the "Today" button, the
Serial Day Number on this page is displayed as noon UTC because that is when it
changes. The Jewish calendar changes at sunset and will be reflected if the UTC
time is after 6 p.m.
? Astronomers, unlike historians, frequently need to do arithmetic with dates.
For example: a double star goes into eclipse every 1583.6 days and its last
mid-eclipse was measured to be on October 17, 2003 at 21:17 UTC. When is the
next? Well, you could get out your calendar and count days, but it's far easier
to convert all the quantities in question to Julian day numbers (serial day
number) and simply add or subtract. Julian days simply enumerate the days and
fraction which have elapsed since the start of the Julian era, which is defined
as beginning at noon on Monday, 1st January of year 4713 B.C.E. in the Julian
calendar. This date is defined in terms of a cycle of years, but has the
additional advantage that all known historical astronomical observations bear
positive Julian day numbers, and periods can be determined and events
extrapolated by simple addition and subtraction. Julian dates are a tad
eccentric in starting at noon, but then so are astronomers (and systems
programmers!)--when you've become accustomed to rising after the "crack of
noon" and doing most of your work when the Sun is down, you appreciate
recording your results in a calendar where the date doesn't change in the
middle of your workday. But even the Julian day convention bears witness to the
eurocentrism of 19th century astronomy--noon at Greenwich is midnight on the
other side of the world. But the Julian day notation is so deeply embedded in
astronomy that it is unlikely to be displaced at any time in the foreseeable
future. It is an ideal system for storing dates in computer programs, free of
cultural bias and discontinuities at various dates, and can be readily
transformed into other calendar systems, as the source code for this page
illustrates. Use Julian days and fractions (stored in 64 bit or longer floating
point numbers) in your programs, and be ready for Y10K, Y100K, and Y1MM!
While any event in recorded human history can be written as a positive Julian
day number, when working with contemporary events all those digits can be
cumbersome. A Modified Julian Day (MJD) is created by subtracting 2400000.5
from a Julian day number, and thus represents the number of days elapsed since
midnight (00:00) Universal Time on November 17, 1858. Modified Julian Days are
widely used to specify the epoch in tables of orbital elements of artificial
Earth satellites. Since no such objects existed prior to October 4, 1957, all
satellite-related MJDs are positive.
U.S. government calendars are printed with the "Julian day" on them, which is
the number of the day of the year. So January 1st would be Julian day 1 and if
it were not a leap year, December 31st would be Julian day 365. This standard
developed after World War II in the ACP-127 format (Allied Communications
The abbreviation A.D. stands for Anno Domini (Latin) - "in the year of the
Lord", meaning the year(s) since Christ's birth. It wasn't adopted until around
386 A.D. when a Catholic monk decided to find out how many years had past since
Christ's birth. Sadly he was 7 years off. So instead of Christ being born in 1
A.D. (because there is no 0 A.D.), Jesus was actually born in 7 B.C. What I
mean is if the monk had found the correct year, this year wouldn't have been
2021, but 2028.
The Christian origin of the designation of years as "B.C." (Before Christ) and
"A.D." (Anno Domini, Latin for "in the year of the Lord") has led some people
to favor a less culturally specific designation. "C.E." stands for "the Common
Era," and "B.C.E." for "Before the Common Era." In practice, the terms are
perfect synonyms for "B.C." and "A.D."
Molad means "new moon" or more accurately, "birth of moon". A.M. is "Anno
Mundi" which means "year of the world" that is the year of creation. (ante) is
before the calendar was implemented, (post) is after the expiration of the
calendar. For example, before the Gregorian calendar was the Julian calendar.
Before the Julian calendar you have the Roman calendar and various other
cultures, but for this program that would be the Jewish calendar.
Tidbit: Most U.S. official certificates, decrees, laws, etc. must
spell out the meaning of A.D. but in English to "in the year of the Lord". So
12-16-2002 (mm-dd-yyyy) on a U.S. certificate would be "This, the Sixteenth day
of December, in the year of the Lord Two Thousand and Two, ..."
The actual repeatable cycle of the Gregorian calendar is 400 Gregorian years.
Hence, the average Gregorian year is 365.2425 days long. That means that the
Gregorian calendar is slower than the mean tropical solar year by about 3 days
in every 10,000 years.
The accuracy of the Hebrew calendar is fixed by the value of the mean lunation
period coupled to the 19 year cycle of 235 lunar months. This is called a molad
(new moon), which is exactly 29 days, 12 hours and 793 halakim (parts), or 3
1\3 seconds. One hour is broken into 1080 halakim. This means that the
computations for a date are so accurate that any mean lunar conjunction can be
found within 1 day in 14,000 years. A 1 day error in 14,000 years. (Gregorian:
1 day per 3300 years error) (Julian: 1 day in 128 years error). That leads to
an average Hebrew year length of 365.2468 days. The mean tropical solar year is
about 365.2422 days. Hence, the average Hebrew year is slower than the average
solar year by about one day in every 216 years. That means that today, we
celebrate the holidays, on average about 8 days later than did our ancestors in
359 AD at the time that the fixed calendar rules were published. This makes the
Hebrew calendar the most accurate in the world because it takes into account
the lunar and solar cycles. Even NASA and the U.S. Naval Observatory has to
throw in a leap second now and again to "fix" the solar calendar. states "Thus, adding 3760 to any Julian/Gregorian year number
after 1 CE will yield the Hebrew year which roughly coincides with that English
year, ending that autumn. (Add 3760 for the year beginning in autumn - e.g. 1 CE
= 3761). Due to the slow drift of the Jewish calendar relative
to the Gregorian calendar, this will be true for about another 20,000 years.
...Due to it's accuracy the total accumulated error ... from its Babylonian
measurement until the present amounts to only about five hours."
According to Jewish tradition, the year 1 of the Jewish calendar was the time
of "waste and void" referred to in Genesis 1:1. Nothing was yet created, and
only a virtual clock started to tick on the first day of that year, heard, as
it were, only by the Creator, on the first day of the week (Sunday) the 24th of
Elul, (22 August 3760 B.C. in the Gregorian calendar). He said "Let there be
light", and He finished the following Sabbath (Saturday) which is the first day
of Tishri, year 2. All Hebrew days begin at sunset (~18:00 hours) which
corresponds to hour zero of the Hebrew calendar's day. The first day of the
Hebrew calendar is Sunday at 11:20:11 P.M. This would actually be Monday,
because the Jewish day is considered to begin at sunset. Since sunset varies,
the day is assumed to begin at 6:00 P.M. for calendar calculation purposes. So,
the first molad was 5 hours 793 halakim after the start of Tishri 1, 0001
(which was Monday September 7, 3761 B.C. by the Gregorian calendar).
The old Roman calendar was very complicated and required a group of men, known
as the pontiffs, to decide when days should be added or removed to keep the
calendar in track with the seasons. This made planning ahead difficult and the
pontiffs were open to bribery in order to prolong the term of elected officials
or hasten elections. In order to avoid these problems Julius Caesar abolished
the use of the lunar year and the intercalary month, and regulated the civil
year entirely by the sun. With the advice and assistance of Sosigenes, he fixed
the mean length of the year at 365 1/4 days, and decreed that every fourth year
should have 366 days, the other years having each 365. In order to restore the
vernal equinox to the 25th of March, the place it occupied in the time of Numa,
he ordered two extraordinary months to be inserted between November and
December in the current year, the first to consist of thirty three, and the
second of thirty-four days. The intercalary month of twenty-three days fell
into the year of course, so that the ancient year of 355 days received an
augmentation of ninety days; and the year on that occasion contained in all 445
days. This was called the last year of confusion. The first Julian year
commenced with the 1st of January of the 46th before the birth of Christ, and
the 708th from the foundation of the city.
In the distribution of the days through the several months, Caesar adopted a
simpler and more commodious arrangement than that which has since prevailed. He
had ordered that the first, third, fifth, seventh, ninth, and eleventh months,
that is January, March, May, July, September and November, should have each
thirty-one days, and the other months thirty, excepting February, which in
common years should have only twenty-nine days, but every fourth year thirty
days. This order was interrupted in 8 B.C. to gratify the vanity of Augustus,
by giving the month bearing his name as many days as July, which was re-named
after the first Caesar during 44 B.C. A day was accordingly taken from February
and given to August; and in order that three months of thirty-one days might
not come together, September and November were reduced to thirty days, and
thirty-one given to October and December.
The additional day which occurred every fourth year was given to February, as
being the shortest month, and was inserted in the calendar between the 24th and
25th day. February having then twenty-nine days, the 25th was the 6th of the
calends of March, sexto calendas; the preceding, which was the additional or
intercalary day, was called bis-sexto calendas,--hence the term bissextile,
which is still employed to distinguish the year of 366 days. The English
denomination of leap year would have been more appropriate if that year had
differed from common years in defect, and contained only 364 days. In the
modern calendar the intercalary day is still added to February, not, however,
between the 24th and 25th, but as the 29th.
In the Julian calendar, the tropical year is approximated as 365 1/4 days =
365.25 days. This gives an error of 1 day in approximately 128 years. The
approximation 365 1/4 is achieved by having 1 leap year every 4 years. The rule
is that every year divisible by 4 is a leap year.
It does not seem to matter how you look at it, figuring out the right date
isn't easy. Something to keep in mind though is that the moon has been moving
away from the earth at about 3.5 cm a year. So the lunar cycle has grown longer
over the years. Not by much, but enough if you wish to be accurate. Twice in
the 1990's that I know of, December 31st has been lengthened by one second.
Even with atomic clocks, no one seems to know what time it is, and that we
never seem to have enough of it. :)
Questions or comments, please contact the