Zeller's congruence not working

[Brain_Child]

im trying to write a program where a user inputs a date and the day of the week is then printed to the screen. so 3/4/2009 would print friday

im using Zeller's congruence (Zeller's congruence - Wikipedia, the free encyclopedia) but it doesnt seem to give the correct result. here's my maths.

   q  = DD; // DD is day entered by the user
   m  = MM; // MM is the month entered by the user
   m  = ( (m+1) * 26 ) / 10;
   K  = YYYY % 100; // YYYY is the year entered by the user
   KK = K / 4;
   J  = YYYY / 100;
   JJ = J / 4;

   DayOfTheWeek = q + m + K + KK + JJ - (2*J);

   DayOfTheWeek = DayOfTheWeek % 7;

then if statements to print out the days with 0 = sat, 1 = sun ... 6 = fri.

it doesnt work for some days eg 1/1/1700 was a friday, but my program will say it is a wednesday. ive tried the other formulae on the wiki page, like changing the -2J to +5j, but doesnt change anything. get the same errors

but it does work for many days, which is why i think the problem may just be something minor

so i was wondering if some fresh eyes could spot the flaw that i cant pick up. thx

[nacho4d]

probably is not the reason... but
wikipedia says something like:
DayOfTheWeek = q + floor(m) + K + floor(KK) + floor(JJ) - (2*J);
and you didn't talk about what type is m, KK and JJ, please make sure you have the right values

[Brain_Child]

probably is not the reason... but
wikipedia says something like:
DayOfTheWeek = q + floor(m) + K + floor(KK) + floor(JJ) - (2*J);
and you didn't talk about what type is m, KK and JJ, please make sure you have the right values

they are all integer values

I thought C automatically floors integer values which is why I did all of the smaller calculations involving flooring first and added everything at the end.

[Adak]

Did you note the last examples in the Wiki article that shows January 2000 should be treated as the 13 month of 1999?

Apparently this is true for January and February, and then on March 1, the Zeller figures demand that to be the *third* month of the current year. So there is no month 1 or month 2 of the current year.

Integer values are truncated, which is what Zeller's equation needs, but watch out for the modulo of a negative number. Such a number must be changed according to the Wiki article.