[Search for users] [Overall Top Noters] [List of all Conferences] [Download this site]

Conference rusure::math

Title:Mathematics at DEC
Moderator:RUSURE::EDP
Created:Mon Feb 03 1986
Last Modified:Fri Jun 06 1997
Last Successful Update:Fri Jun 06 1997
Number of topics:2083
Total number of notes:14613

784.0. "Leap" by SRFSUP::FRIEDMAN () Fri Nov 06 1987 20:54

    Will the year 2100 be a leap year?  Why or why not?
T.RTitleUserPersonal
Name
DateLines
784.1NoZFC::DERAMODaniel V. D'EramoFri Nov 06 1987 21:3122
    It won't be.  The rule is that if the year is divisible by 4 then
    it is a leap year -- UNLESS it is also divisible by 100 when it
    is not a leap year -- UNLESS it is also divisible by 400 when it
    is a leap year.
    
    Examples:
    
    1896 1900 1904    1999 2000 2001     2096 2100 2104
     YES  NO   YES     NO   YES  NO       YES  NO   YES
    
    If a year was exactly 365.25 days then it would work to have leap
    years every fourth year.  However, the year is not 365.25 days,
    so every now and then they leave out a "fourth year" -- those that
    are divisible by 100.  But that is an over-correction, so every
    400-th year is kept as a leap year.  So over 400 years, there are
    three fewer leap days than if every fourth year were a leap year.
    
    I think they now adjust atomic clocks a fraction of a second every
    six months or so to keep things lined up.  Otherwise, for example,
    they might have to skip leap day every 40,000 years [made up number].
    
    Dan
784.2side effectVINO::JMUNZERMon Nov 09 1987 12:243
    See note # 511.
    
    John
784.3EAGLE1::DANTOWITZDavid - BXB1-1/E11 DTN: 293-5356Mon Nov 09 1987 19:02186
            <<< UFP::SYS$SYSDEVICE:[NOTES$LIBRARY]HACKERS.NOTE;1 >>>
================================================================================
Note 216.13              OS With Small DateTime Location                13 of 22
KOALA::ROBINS "Scott A. Robins""                    177 lines   5-MAR-1986 11:39
                             -< Here's stan's SPR >-
--------------------------------------------------------------------------------

Subj:	Stanley hacks a user with a Rabinowitzian SPR response
Subj:	a sledgehammer...
Subj:	AND 2000 is the last year of the 20th Century not the first of the 21st
Subj:	Fixed in next century
Subj:	SPR response on leaping ahead to the year 2000
Subj:	Calendar SPR
Subj:	try again.....
Subject: My SPR answer (for your review)

                   ******DEC INTERNAL USE ONLY******            
 
SPR NUMBER:                  11-60903

ANSWER CATEGORY:             UE
MAINTENANCE HOURS:           1
DUPLICATE PROBLEM:           N
DUPLICATE SPR NUMBER(S):     
 
OPERATING SYSTEM:            VAX/VMS             
O.S. VERSION:                V3.2
PRODUCT:                     VAX/VMS
PRODUCT VERSION:             V3.2
COMPONENT:                   Run-Time Library
SUB-COMPONENT:               LIB$ routines
 
DATE ANSWERED:               13-Oct-1983

MAINTAINER:                  Stanley Rabinowitz
 
ATTACHMENT:                  N

PUBLICATION INSTRUCTIONS:    N

SPR PROBLEM ABSTRACT:        User claims year 2000 should not be a leap year.

TITLE:                       -
PUBLICATIONS:                -
ADDITIONAL O.S. VERSIONS:
ADDITIONAL PRODUCT VERSIONS:
COMPONENT SEQUENCE NUMBER:   
SUPERSEDES:                  
TYPE OF ARTICLE:             

                            ANSWER CATEGORIES

CG=1=CORRECTION GIVEN       RS=5=RESTRICTION              SG=9=SUGGESTION
FN=2=FIXED IN NEXT RELEASE  CS=6=CUSTOMER SUPPORTED       IQ=10=INQUIRY
DE=3=DOCUMENTATION ERROR    NR=7=NON-REPRODUCIBLE         HW=11=HARDWARE
UE=4=USER ERROR             II=8=INSUFFICIENT INFORMATION

                            TYPE OF ARTICLE

F=OPTIONAL FEATURE PATCH    N=NOTE
M=MANDATORY PATCH           R=RESTRICTION

                         FOR MAINTENANCE USE





                     ******END OF DEC USE ONLY******
                            D I G I T A L

                           SPR ANSWER FORM

SPR NO. 11-60903


           SYSTEM   VERSION   PRODUCT   VERSION   COMPONENT
SOFTWARE:  VAX/VMS  V3.2      VAX/VMS   V3.2      Run-Time Library



PROBLEM:

The LIB$DAY Run-Time Library service "incorrectly"  assumes  the  year
2000 is a leap year.


RESPONSE:

Thank you for your forward-looking SPR.

Various system services, such as SYS$ASCTIM assume that the year  2000
will  be  a  leap  year.   Although one can never be sure of what will
happen at some future time, there is strong historical  precedent  for
presuming  that the present Gregorian calendar will still be in affect
by the year 2000.  Since we also hope that VMS will still be around by
then, we have chosen to adhere to these precedents.

The purpose of a calendar is to reckon time in advance,  to  show  how
many  days  have  to  elapse  until a certain event takes place in the
future, such as the harvest or the release of VMS  V4.   The  earliest
calendars,  naturally,  were  crude  and  tended  to be based upon the
seasons or the lunar cycle.

The calendar of the Assyrians, for example, was based upon the  phases
of  the  moon.  They knew that a lunation (the time from one full moon
to the next) was 29 1/2 days long, so their lunar year had a  duration
of  364  days.   This  fell  short of the solar year by about 11 days.
(The exact time for the solar year is approximately 365 days, 5 hours,
48  minutes,  and  46  seconds.)  After 3 years, such a lunar calendar
would be off by a whole month, so the Assyrians added an  extra  month
from  time  to time to keep their calendar in synchronization with the
seasons.

The best approximation that was possible in antiquity  was  a  19-year
period, with 7 of these 19 years having 13 months (leap months).  This
scheme was adopted as the basis for the religious calendar used by the
Jews.   (The  Arabs  also  used  this  calendar until Mohammed forbade
shifting from 12 months to 13 months.)

When Rome emerged as a world  power,  the  difficulties  of  making  a
calendar  were  well  known,  but  the  Romans complicated their lives
because of their superstition that even numbers were  unlucky.   Hence
their  months were 29 or 31 days long, with the exception of February,
which had 28 days.  Every second year, the Roman calendar included  an
extra  month  called  Mercedonius of 22 or 23 days to keep up with the
solar year.

Even this algorithm was very poor, so that in 45 BC,  Caesar,  advised
by  the  astronomer Sosigenes, ordered a sweeping reform.  By imperial
decree, one year was made 445 days long to bring the calendar back  in
step  with  the  seasons.  The new calendar, similar to the one we now
use was called the Julian calendar (named after Julius Caesar).   It's
months  were  30 or 31 days in length and every fourth year was made a
leap year (having 366 days).  Caesar also decreed that the year  would
start with the first of January, not the vernal equinox in late March.

Caesar's year was 11 1/2 minutes short of the calculations recommended
by  Sosigenes  and  eventually the date of the vernal equinox began to
drift.  Roger Bacon became alarmed and sent a note to Pope Clement IV,
who  apparently  was  not  impressed.   Pope  Sixtus  IV  later became
convinced that  another  reform  was  needed  and  called  the  German
astronomer,  Regiomontanus,  to  Rome  to  advise him.  Unfortunately,
Regiomontanus died of the plague shortly thereafter and the plans died
as well.

In 1545, the Council of Trent authorized Pope Gregory XIII  to  reform
the  calendar  once  more.   Most of the mathematical work was done by
Father Christopher Clavius, S.J.  The immediate  correction  that  was
adopted  was  that Thursday, October 4, 1582 was to be the last day of
the Julian calendar.  The next  day  was  Friday,  with  the  date  of
October  15.   For  long  range  accuracy,  a formula suggested by the
Vatican librarian Aloysius Giglio was adopted.   It  said  that  every
fourth  year  is  a  leap  year  except for century years that are not
divisible by 400.  Thus 1700, 1800 and 1900 would not be  leap  years,
but  2000  would  be a leap year since 2000 is divisible by 400.  This
rule eliminates 3 leap years every 4 centuries,  making  the  calendar
sufficiently  correct  for  most  ordinary purposes.  This calendar is
known as the Gregorian calendar and is the one that we now use  today.
(It  is  interesting  to note that in 1582, all the Protestant princes
ignored the papal decree and so many countries continued  to  use  the
Julian  calendar  until either 1698 or 1752.  In Russia, it needed the
revolution to introduce the Gregorian calendar in 1918.)

This explains why VMS chooses to treat the year 2000 as a leap year.

Despite the great accuracy of the Gregorian calendar, it  still  falls
behind very slightly every few years.  If you are very concerned about
this problem, we suggest that you tune in  short  wave  radio  station
WWV,  which  broadcasts  official  time  signals for use in the United
States.  About once every 3 years, they declare a leap second at which
time  you  should be careful to adjust your system clock.  If you have
trouble picking up their signals, we suggest you  purchase  an  atomic
clock (not manufactured by Digital and not a VAX option at this time).










                         END OF SPR RESPONSE

784.4I couldn't resist ...ZFC::DERAMODaniel V. D'EramoMon Nov 09 1987 21:479
    >>   The immediate  correction  that  was adopted  was  that
    >>   Thursday, October 4, 1582 was to be the last day of
    >>   the Julian calendar.
    
         Or, as they explained at the time ...
    
              10/4, over and out!
    
         :^)