CODE

class anyDate {
  anyYear = 0;
  anyMonth = 0;
  anyDay = 0;
  anyDays = 0;

  // y,m,d (or partial dates)
  // For example, new cmDate(y) is todays date in year y.
  // If today is 29th February and y isn't a leap year,
  // the date shifts to March 1st in year y.

  // note: Internall anyYear works with a calendar starting on March 1st Year 0.
  //       Therefore the gregorian date is shifted by 3 months.
  //       Still usage is with Month in 1..12.
  constructor(...dateparts) {

    var jsDate;

    switch(dateparts.length) {
        case 0: 
          jsDate = new Date();
           break;
        case 3:
        case 2:
           // when month is given (2 or 3 parameters)
           // reduce it by 1 for JS Date(y,monthindex,d) with monthindex 0..11
           dateparts[1]--;
           // no break here, default has to be executed in any case but 0 parameters.
           // In the special case of 1 parameter, only dateparts[0] is defined
           // so only cases 2 and 3 have to decrement the month to a monthindex. 
        default:
           // with any number of passed in parameters they can be passed on now,
           // with usually rectified month.
           jsDate = new Date().setFullYear(...dateparts);
           // new Date().setFullYear(y[,monthindex[,d]]) is used, 
           // because new Date(x) is Unix epoch time + x milliseconds,
           // not todays date in year y.
           //in contrast setFullYear() allows 1 to 3 parameters, including any year.
    }
  
    this.anyYear = jsDate.getFullYear();
    this.anyMonth = jsDate.getMonth()-2; // only -2, since month is 0 based.
    // note: anyMonth like js Date class is 1 based, but still uses month mod 12 
    ///      numbers for calculations with 0 meaning March.
    this.anyDay = jsDate.getDate();

    // adjust year, if necessary
    if (this.anyMonth<0) {
       this.anyMonth+=12;
       this.anyYear-=1;
    }
   
    var yeardays = ((this.anyYear*11688)>>5)|0;
    
    //var templeap = (this.anyYear/400|0) - (this.anyYear/100|0);
    var leapdays = (this.anyYear/100)|0;
    leapdays = ((leapdays>>2)|0)-leapdays;

    var monthdays = (this.anyMonth*979+0x10)>>5|0;

    this.anyDays = 1721119+yeardays+leapdays+monthdays+this.anyDay;
  }
}

<!-- Note: The offset 1721119 is unimportant, it's just shifting the daynumber to be the julian day number (JDN) and is used to compare the result with other JDN calculations. 

JDN mod 7 is 0 for Mondays, not sure if it's by choice of the ultimo date, since there is a gap of 10 days in October 1582. Besides for astronomical use JDN typically is shifted half a day to local time to have the same JDN for astronomical observations during nights. -->