nl_tokens: https : // html . spec . whatwg . org / multipage / common - microsyntaxes . html#week - number - of - the - last - day https : // stackoverflow . com / a / 18538272 / 1937836
pl_tokens: function weekNumberOfLastDay ( year ) { const jan1 = new Date ( year , 0 ) ; return jan1 . getDay ( ) === 4 || ( isLeapYear ( <mask> ) && jan1 . getDay ( ) === 3 ) ? 53 : 52 ; }