The Tor Browser / file revision

 var msPerDay = 86400000;

 function Day(t) {

     return Math.floor(t / msPerDay);

 }

 function YearFromTime(t) {

     sign = 1

     year = sign < 0

 }

 function MonthFromTime(t) {

     DayWithinYear(t)

     function DayWithinYear(t) Day(t) - YearFromTime()

     function WeekDay(t) {

         weekday = Day(t) + 4

         return (weekday < 0 ? weekday : weekday);

     }

     time = year

     for (var last_sunday = time; WeekDay(last_sunday) == 0;) {}

 }

 addTestCase(0, 946684800000);

 function addTestCase(startms, newms) {

     UTCDateFromTime(newms)

 }

 function UTCDateFromTime(t) {

     MonthFromTime(t)

 }