I'm having trouble with modulus giving me the wrong answer.

It's taking 734170 and dividing by 7 and is coming up with a remainder of 3, which is wrong.

Here's the bugged out code:


Code:
 
// firstDayCount is set to 734190 
// and dayNum is set to 21 at this point

firstDayCount = firstDayCount - (dayNum - 1) ;

firstDayCount = firstDayCount % 7;
 
// for some reason, after this line of code it comes the answer of 3 when it should be 4
I was wondering if there is a special way modulus works that I'm not fully aware of that could be causing it to come up with this answer.