date-mini.js

function Modulo(value, remainder) {
  var mod = value % remainder;
  // Guard against returning -0.
  if (mod == 0) return 0;
  return mod >= 0 ? mod : mod + remainder;
}


function WeekDay(time) {
  return Modulo(Math.floor(time / 86400000) + 4, 7);
}

var t1 = Date.now();
var t2 = Date.now();

console.log(t1);
console.log(t2);

console.log(WeekDay(t1));
console.log(WeekDay(t1));

console.log(WeekDay(t2));
console.log(WeekDay(t2));