- Dear Rob,

with the help of four offset method i m not getting correct day for 4/november/1976 and 12/june/1976.Can you kindly tell me the way.Please give some examples of date in first week like 1 ,2,3 etc.

Regards

Deepak

On Tue, 31 Jan 2006 Ulrich Voigt wrote :>Dear Robert,

[Non-text portions of this message have been removed]

>I would not call this four-offset method "standard" because the numbering 1

>= mo, 2 = tue etc. breaks with a tradition which goes well back to the

>middle ages.

>I prefer to keep in contact with tradition and stick to 1 = su, 2 = mo

>(like Portuguese mo = secunda feira,... , fri = sexta feira).

>This concerns only the numbers associated with the months.

>Ulrich

>

>On 1/29/06, rob221b <rob221b@...> wrote:

> >

> > Thank YOU Issam - I don't think you have been wasting your time in

> > discovering a calendar method. By "exploring all those combinations"

> > you must have acquired a genuine feel for the calendar, something

> > that is lacking in someone who has simply learnt a method by rote

> > from a book or from a message posted to this group without any

> > understanding of why the method works.

> >

> > 31-12-2999??? Is there a shortage of dentists in Spain as well?

> >

> > Best wishes,

> > Robert

> >

> >

> > --- In MentalCalculation@yahoogroups.com, issam khneisser

> > <issamn_1@y...> wrote:

> > >

> > > hi rob,

> > >

> > > WOW, thank you from every body in this group for the

> > information. but unfortunately, i had discover that alone. i should

> > had waited till i read all these details from you.

> > > and not waisted my time to explore all those combinations.

> > >

> > > using your method to compute Alberto Question:

> > > Why 31-12-2999 is a tuesday?

> > >

> > > it is true to be a tuesday just read Robert explanation below

> > >

> > > best regards

> > > Issam KHNEISSER

> > >

> > >

> > > rob221b <rob221b@y...> wrote:

> > > Hi Jan,

> > >

> > > By "standard four-offset method" I mean the method of assigning

> > > offsets to the day of the month, the month, the century and the

> > year,

> > > and adding them up to get the offset for the day of the week. You

> > know

> > > all about this of course, but in case anyone is not familiar,

> > here's a

> > > summary of the principles underlying the method used by most

> > calendar

> > > calculators:

> > >

> > > Base date: Sunday 0 January 1900

> > > 1900 is convenient for the base century/year because most people

> > > living were born in the 20th century, and so the "On which day of

> > the

> > > week was I born?" questions come mainly from them.

> > > There is no 0th day of the month, but that doesn't matter (really

> > the

> > > base date is the 31st Dec 1899, but better to have zero point

> > > comprising zeros).

> > >

> > > The 0th is the base day of the month, and has offset zero.

> > > If the 0th of a particular month is a Tuesday, then the 24th day

> > of

> > > the same month must be a Friday, because 24 mod 7 = 3, and so the

> > 24th

> > > falls on a day of the week that is 3 days later than the 0th. The

> > 24th

> > > is therefore assigned an offset 3 relative to the base day of the

> > > month (the 0th).

> > >

> > > January, the base month has offset zero.

> > > January has 31 days, so February is assigned offset 3 (31 mod 7 =

> > 3),

> > > i.e. date in Feb is 3 days later than the same date in January.

> > > Example: Today is Saturday 21 Jan, so 21 Feb this year will be a

> > > Tuesday.

> > > February has 28 days (for now ignoring leap years), so a date in

> > March

> > > will fall on the same day of the week as it does in February (28

> > mod 7

> > > = 0). Therefore March has offset zero relative to February and an

> > > offset 3 relative to January. The 21 Mar this year will be a

> > Tuesday,

> > > same as February.

> > > March has 31 days and 31 mod 7 = 3, so dates in April fall on day

> > of

> > > the week that is 3 days later than the same date in March. So

> > April

> > > has offset 3 relative to March, and therefore an offset 6 relative

> > to

> > > January. So 21 April this year will be a Friday.

> > > Continuing this, offsets relative to January can be defined for

> > every

> > > month:

> > >

> > > Jan 0

> > > Feb 3

> > > Mar 3

> > > Apr 6

> > > May 1

> > > Jun 4

> > > Jul 6

> > > Aug 2

> > > Sep 5

> > > Oct 0

> > > Nov 3

> > > Dec 5

> > >

> > > It is not necessary to commit this table to memory if you know the

> > > number of days in a month, but it speeds up the calc if it IS

> > > memorised. Possibly at first just remember (besides Jan 0) two or

> > > three offsets and generate the rest (I remember May 1,

> > because "May

> > > day" and Oct 0 because Oct begins with 0). Example, if know offset

> > for

> > > October is zero and know September has 30 days, then offset for

> > > September must be 5, since 30 mod 7 + 5 = 0.

> > >

> > > The base year of any century is 00, which is assigned offset 0.

> > > 365 mod 7 =1, so if a certain date one year falls on a Friday,

> > then

> > > the same date will fall on a Saturday the following year. If it

> > were

> > > not for leap years, then the year offsets would simply increase by

> > 1

> > > each year: 00 would have offset 0, 01 would have offset 1, 02

> > would

> > > have offset 2. 03 offset 3, 04 offset 4, and so on up to 99, which

> > has

> > > offset 1 (=99 mod 7). Leap years complicate matters slightly and

> > the

> > > simple offset is increased by 1 every February the 29th. Rather

> > than

> > > have two offsets for leap years (one applicable up to Feb 29th and

> > one

> > > after Feb 29th) it is simpler to have one offset for the whole

> > leap

> > > year, and since Jan-Feb is a much shorter period than Mar-Dec,

> > then it

> > > is better to use the offset applicable to the latter period and

> > treat

> > > Jan/Feb in Leap years as a special case (deducting one from the

> > final

> > > offset more on this later).

> > >

> > > The offsets for the years are therefore:

> > >

> > > 00 0

> > > 01 1

> > > 02 2

> > > 03 3

> > > 04 5

> > > 05 6

> > > 06 0

> > > 07 1

> > > 08 3

> > > 09 4

> > > .

> > > .

> > > .

> > > 98 3

> > > 99 4

> > >

> > > I should have emphasised earlier that Offset is identical to

> > (Offset

> > > mod 7), because the days have a 7 day cycle -a week, and that all

> > > arithmetic can be performed modulo 7.

> > >

> > > It is of course very easy to calculate the year offsets as

> > explained

> > > above instead of memorizing them -for year Y, evaluate (Y mod 7 +

> > Y

> > > div 4) mod 7, or (Y + Y div 4) mod 7 or an equivalent expression,

> > but

> > > the fastest calculations are those that involve the least

> > calculating.

> > > Also can exploit the fact that Y+28 has the same year offset as Y

> > (28

> > > is the smallest multiple of 7 and 4).

> > >

> > > The base century is the 20th century (or the 1900s), which has

> > offset

> > > 0.

> > > A date in the 21st century falls on day of the week that is one

> > day

> > > before the same date in the 20th century. To see why, look at the

> > > above table of year offsets. The year 99 has an offset 4. If

> > continue

> > > the table, the next year year 100 would have offset 6, so the

> > year

> > > 2000 has offset 6 relative to the base year, 1900. This means the

> > 21st

> > > century has offset 6 relative to the 20th century. In the same

> > way,

> > > offsets can be assigned to other centuries.

> > > If a year cc00 is such that cc is not divisible by 4, then it is

> > not a

> > > leap year. So, 1700,1800,1900, 2100, 2200, 2300 are not leap

> > years,

> > > but 1600, 2000 and 2400 are leap years. The offsets for the

> > centuries

> > > are therefore:

> > >

> > > 20 0

> > > 21 6

> > > 22 4

> > > 23 2

> > > 24 0

> > > 25 6

> > > and the pattern continues.

> > >

> > > The pattern extends in the other direction also, so the 19th

> > century

> > > has offset 2, the 18th century has offset 4, etc. The calendar

> > clearly

> > > repeats itself every 400 years.

> > >

> > >

> > > With all 4 offsets now defined, an example is in order:

> > >

> > > 16 April 1758

> > > Day of the month 16 has offset 2

> > > Month April has offset 6

> > > Century 18 (i.e. the 1700s) has offset 4

> > > Year 58 has offset 2

> > >

> > > Total offset 0, i.e. the day of the week is zero days after the

> > day of

> > > the week of our base date of 0 January 1900 a Sunday.

> > >

> > >

> > > An example to illustrate special case for Jan/Feb in Leap Year..

> > >

> > > 11 February 13852

> > > 11 has offset 4

> > > Month Feb has offset 3

> > > Century 139 (the 13800s) has offset 2 (use fact that calendar

> > repeats

> > > every 400 years, so just subtract multiple of 4 from 38 to get

> > near

> > > familiar territory here 38 20 = 18, so offset is same as that

> > for

> > > the 1800s, i.e. 2)

> > > Year 52 has offset 2

> > >

> > > Total offset 4, so Thursday. BUT 52 is a leap year and month is

> > > February, so, as explained before, should be day earlier, i.e.

> > > Wednesday. Wednesday is correct.

> > >

> > >

> > > That's the theory. Here's the practice..

> > >

> > > If the date is proposed verbally (not written), then the

> > calculation

> > > can begin before the full date has been given.

> > > Example:

> > > 22 July 1960

> > > When hear "22" immediately have in mind 1 (offset for 22)

> > > When hear "July", calculate in no time at all 0 (either do 1+6 or

> > 1-1,

> > > i.e. offset for July is either 6 or 1, they are equal modulo 7).

> > > When hear "19", do nothing because that's the base century, so

> > total

> > > offset will just be whatever the year offset is (month is not

> > Jan/Feb,

> > > so no leap year correction to make whatever the year turns out to

> > be).

> > > When hear "60", can say immediate "Friday" (year 60 has offset 5).

> > >

> > > If the date is proposed in written form, then it might be better

> > not

> > > to cast out sevens at all (the final offset will never exceed 49

> > and

> > > with practice the association `offset-day', e.g. 49-Sunday, will

> > > become automatic). I don't know this for sure, since I

> > automatically

> > > cast out sevens cannot change a habit of 20 years!

> > >

> > >

> > > There are a number of advantages in having the year offsets

> > memorised

> > > rather than calculate them. They can be used as a look-up table

> > when

> > > answering questions such as, in which years between 1920 and 1975

> > did

> > > the 18th of August fall on a Friday?

> > >

> > > Rain man? It is easy, especially if you know the magic sequence 6,

> > 5,

> > > 6, 11, 6, 5, 6, 11, ...

> > >

> > > 18 August has offset 6 (4+2).

> > > If the Year offset is Y, then 6 + Y = 5 (Friday).

> > > So Y is 6.

> > >

> > > Now we search for the first 6 in our memorised table of year

> > offsets

> > > starting with the year 20 (i.e. the year 1920).

> > > Year 20 has offset 4

> > > Year 21 has offset 5

> > > Year 22 has offset 6

> > > So the first year after 1920 for which 18 Aug was Friday, is 1922.

> > > Now, years, including leap years, with identical calendars follow

> > the

> > > sequence 6, 11, 6, 5, 6, 11, 6, 5, 11. Example: 1900, 1906, 1917,

> > > 1923, 1928, 1934,..., so the next year after 1920 for which the

> > 18th

> > > Aug fell on a Friday is either 1927, 1928 or 1933. We test first

> > the

> > > offset for the year 27, which is 5. We can then reel off all the

> > years

> > > (the next must be 6 years later, then 11 years, and so on

> > according to

> > > 5 6 11 6 5 6 11).

> > > Hence, the years are:

> > > 1922, 1927, 1933, 1944, 1950, 1955, 1961, 1972.

> > > Notice the pattern 5, 6, 11, 6, 5, 6, 11.

> > >

> > > If the first offset had been a 6 instead of 5, then the second

> > year in

> > > the sequence must be identified from the look-up table, since 6

> > can be

> > > followed by either 5 or 11.

> > >

> > >

> > > And that's the standard four-offset method.

> > >

> > > Robert

> > >

> > >

> > >

> > > --- In MentalCalculation@yahoogroups.com, "Jan van Koningsveld"

> > > <jan.van.koningsveld@e...> wrote:

> > > >

> > > > Hi Robert,

> > > >

> > > > what exactly is the standard four-offset method ?

> > > >

> > > > Jan

> > > >

> > > >

> > > > After experimenting a little speaking the days insytead of

> > writing

> > > > them, I discovered that a score of 40+ can be achieved using

> > the

> > > > standard four-offset method, after all. RobF

> > > >

> > > >

> > > >

> > > > [Non-text portions of this message have been removed]

> > > >

> > >

> > >

> > >

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- Emmanuel/DATA <emmanuel_dubrulle@...> wrote

1676, February 23

16 from 18 gives "2"

76 is 6 dozen and 4, giving 10; plus 1 gives 11, i.e. "4".

Total "6"

The item for February is "3". Total 9, i.e. "2"

23 gives "2". Total "4"

Correction for Leap Year gives "3". Answer, "Wednesday"

Dear Emmanuel

I am not sur that 1676 february 23 is a wednesday

Please review your method

best regards

Issam KHNEISSER

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