CHAP 2[C]d Finding day on first of Jan on a particular year

According to the Gregorian calendar, it was Monday on the date 01/01/1900. If any year is input through the keyboard write a program to find out what is the day on 1st January of this year.

void main()
{
float days;
int year,diff,leap,type;
long int days1;
printf("\nInput the year");
scanf("%d",&year);
year=year-1;
diff=year-1900;

/* The line year=year-1 was written because we are finding the days before
that particular year not that full year as the required date is 01/01/year.

In days... 365 was added because the year 1900 has to be taken into account
as 1900-1904 is not 1904-1900=4years but is 5 years. 1900 is not a leap year.

In days... addition of one was added because the first day of the year
(1st of Jan) has to be accounted for*/

if(diff<100)
{
leap =diff/4;
days=(366.0*leap)+((diff-leap)*365+365+1);
days1=days;
type=days1%7;
}

if(diff>=100)
{
leap = (diff/4)-(diff/100)+1+((year-2000)/400);
days=(366.0*leap)+((diff-leap)*365+365+1);
days1=days;
type=days1%7;
}

if(type==0)
printf("Sunday");
if(type==1)
printf("Monday");
if(type==2)
printf("Tuesday");
if(type==3)
printf("Wednesday");
if(type==4)
printf("Thursday");
if(type==5)
printf("Friday");
if(type==6)
printf("Saturday");

printf("\nThe leap is %d\nThe days is %f\nThe type is %d",leap,days,type);

}