lib/rrule.js
function wk(dt)  {
    var jan1 = new Date(Date.UTC(y(dt), 0, 1));
    return trunc(daydiff(jan1, dt)/7);
}
function wk(dt)  {
    var jan1 = new Date(Date.UTC(y(dt), 0, 1));
    return trunc(daydiff(jan1, dt)/7);
}
function wk ( dt ) { var jan1 = new Date ( Date . UTC ( y ( dt ) , 0 , 1 ) ) ; return trunc ( daydiff ( jan1 , dt ) / 7 ) ; }
Week of year
Week of year