monday2.js - What the hell are you doing?

myModify = document.lastModified;

myD = new Date(myModify);
myYear = myD.getYear();
myYear4 = (myYear < 2000) ? myYear+1900 : myYear;
myMonth = myD.getMonth() + 1;
myDate = myD.getDate();
myDay = myD.getDay();

monthdays = new Array(31,28,31,30,31,30,31,31,30,31,30,31);

if(((myYear%4==0)&&(myYear%100==0))||(myYear%400==0)){
	monthdays[1]=29;
}

switch(myDay){
	case 0:
		myD.setDate(myD.getDate() - 6);
	break;
	case 1:
		myD.setDate(myD.getDate());
	break;
	case 2:
		myD.setDate(myD.getDate() - 1);
	break;
	case 3:
		myD.setDate(myD.getDate() - 2);
	break;
	case 4:
		myD.setDate(myD.getDate() - 3);
	break;
	case 5:
		myD.setDate(myD.getDate() - 4);
	break;
	case 6:
		myD.setDate(myD.getDate() - 5);
	break;
}

document.write('更新日：' + myD.getFullYear() + '年' + (1 + myD.getMonth()) + '月' + myD.getDate() + '日');