for (var j = 0; j < 7; ++j) {

for (var i = 0; i < 6; ++i) {

    // Bind calendar to dateattribute widgets
    cal = new Calendar(false, null, changeWidget, closeHandler);
    // Bind calendar to textfield.
    cal = new Calendar(false, null, changeTextField, closeHandler);

let monthsNames = [
  "January",
  "February",
  "March",
  "April",
  "May",
  "June",
  "July",
  "August",
  "September",
  "October",
  "November",
  "December"
];
let daysInMonth = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
let daysNames = [
  "Monday",
  "Tuesday",
  "Wednesday",
  "Thursday",
  "Friday",
  "Saturday",
  "Sunday"
];

let today = new Date();
let g_currentMonth = today.getMonth();
let g_currentYear = today.getFullYear();
let g_startDay = (new Date(today.getFullYear(), today.getMonth(), 1).getDay() + 6) % 7; //first day in current month

function showCalendar(currentMonth, currentYear, startDay) {
  clearCalendarView();
  let currentDay = getCurrentDay();

  let calendar = document.createElement("table");
  calendar.classList.add("calendar");
  calendar.id = "calendar_id";

  let content = document.getElementById("content");

  let calendarHeader = createHeader();
  calendar.appendChild(calendarHeader);

  let info = createInfo(currentMonth, currentYear);
  info.id = "info_id";
  content.appendChild(info);


  let days = createDaysForMonth(monthsNames[currentMonth], daysNames[startDay], currentDay);
  calendar.appendChild(days);

  content.appendChild(calendar);
}

function getCurrentDay() {
  let today = new Date();
  let thisMonth = today.getMonth();
  let thisYear = today.getFullYear();
  if (thisMonth === g_currentMonth && thisYear === g_currentYear) {
     return today.getDate();
  } 
}

function clearCalendarView() {
  var el = document.getElementById("calendar_id");
  if (el) {
    el.remove();
  }

  var info = document.getElementById("info_id");
  if (info) {
    info.remove();
  }
}

function createHeader() {
  let thead = document.createElement("tr");
  daysNames.forEach(day => {
    let cell = document.createElement("th");
    cell.textContent = day;
    thead.appendChild(cell);
  });
  return thead;
}

function createInfo(currentMonth, currentYear) {
  const result = document.createElement("div");
  result.textContent = (monthsNames[currentMonth]) + '  ' + currentYear;
  result.classList.add("resultMonthAndYear");
  return result;
}

function createDaysForMonth(monthName, startingDay, currentDay) {
  let tbody = document.createElement("tbody");
  let count = 1;

  let firstRow = createFirstRow(startingDay, currentDay);
  tbody.appendChild(firstRow);

  let index = daysNames.indexOf(startingDay);
  let nextMonday = 8 - index; // 7 - index + 1

  let lastMonday = createMiddleRows(tbody, nextMonday, monthName, currentDay);

  let lastRow = createLastRow(lastMonday, monthName, currentDay);
  tbody.appendChild(lastRow);

  return tbody;
}

function createFirstRow(startingDay, currentDay) {
  let row = document.createElement("tr");
  let start = daysNames.indexOf(startingDay);

  for (i = 0; i < start; i++) {
    let cell = document.createElement("td");
    row.appendChild(cell);
  }

  let count = 1;
  for (i = start; i < 7; i++) {
    let cell = document.createElement("td");
    cell.textContent = count;
    row.appendChild(cell);
    if (count === currentDay) {
      cell.classList.add("currentDay");
    }
    count++;
  }
  return row;
}

function createMiddleRows(tbody, startingDay, monthName, currentDay) {
  let monthIndex = monthsNames.indexOf(monthName);
  let daysInCurrentMonth = daysInMonth[monthIndex];
  let count = startingDay;

  while (count + 6 < daysInCurrentMonth) {
    let row = document.createElement("tr");
    for (i = 0; i < 7; i++) {
      let cell = document.createElement("td");
      cell.textContent = count;
      row.appendChild(cell);
      if (count === currentDay) {
        cell.classList.add("currentDay");
      }
      count++;
    }
    tbody.appendChild(row);
  }
  return count;
}

function createLastRow(startDay, monthName, currentDay) {
  let monthIndex = monthsNames.indexOf(monthName);
  let daysInCurrentMonth = daysInMonth[monthIndex];

  let row = document.createElement("tr");
  let count = 0;
  for (i = startDay; i <= daysInCurrentMonth; i++) {
    let cell = document.createElement("td");
    cell.textContent = i;
    row.appendChild(cell);
    if (i === currentDay) {
      cell.classList.add("currentDay");
    }
    count++;
  }
  for (i = count; i < 7; i++) {
    let cell = document.createElement("td");
    row.appendChild(cell);
  }
  return row;
}

function previous() {
  g_currentMonth--;
  if (g_currentMonth < 0) {
    g_currentMonth = 11;
    g_currentYear--;
  }
  g_startDay = (new Date(g_currentYear, g_currentMonth, 1).getDay() + 6) % 7;;
  showCalendar(g_currentMonth, g_currentYear, g_startDay);
}

function next() {
  g_currentMonth++;
  if (g_currentMonth > 11) {
    g_currentMonth = 0;
    g_currentYear++;
  }
  g_startDay = (new Date(g_currentYear, g_currentMonth, 1).getDay() + 6) % 7;
  showCalendar(g_currentMonth, g_currentYear, g_startDay);
}

//*bind execution logic with buttons
let buttonPrev = document.getElementById("previous");
buttonPrev.addEventListener("click", previous);

let buttonNext = document.getElementById("next");
buttonNext.addEventListener("click", next);

showCalendar(g_currentMonth, g_currentYear, g_startDay); //calling the function

<SCRIPT LANGUAGE="JavaScript">

/*
Any-Month calendar script- Rob Patrick (rpatrick@mit.edu)
Script featured on and available at:
http://www.javascriptkit.com/
*/

function setToday() {
var now   = new Date();
var day   = now.getDate();
var month = now.getMonth();
var year  = now.getYear();
if (year < 2000)    // Y2K Fix, Isaac Powell
year = year + 1900; // http://onyx.idbsu.edu/~ipowell
this.focusDay = day;
document.calControl.month.selectedIndex = month;
document.calControl.year.value = year;
displayCalendar(month, year);
}
function isFourDigitYear(year) {
if (year.length != 4) {
alert ("Sorry, the year must be four-digits in length.");
document.calControl.year.select();
document.calControl.year.focus();
} else { return true; }
}
function selectDate() {
var year  = document.calControl.year.value;
if (isFourDigitYear(year)) {
var day   = 0;
var month = document.calControl.month.selectedIndex;
displayCalendar(month, year);
    }
}

function setPreviousYear() {
var year  = document.calControl.year.value;
if (isFourDigitYear(year)) {
var day   = 0;
var month = document.calControl.month.selectedIndex;
year--;
document.calControl.year.value = year;
displayCalendar(month, year);
   }
}
function setPreviousMonth() {
var year  = document.calControl.year.value;
if (isFourDigitYear(year)) {
var day   = 0;
var month = document.calControl.month.selectedIndex;
if (month == 0) {
month = 11;
if (year > 1000) {
year--;
document.calControl.year.value = year;
}
} else { month--; }
document.calControl.month.selectedIndex = month;
displayCalendar(month, year);
   }
}
function setNextMonth() {
var year  = document.calControl.year.value;
if (isFourDigitYear(year)) {
var day   = 0;
var month = document.calControl.month.selectedIndex;
if (month == 11) {
month = 0;
year++;
document.calControl.year.value = year;
} else { month++; }
document.calControl.month.selectedIndex = month;
displayCalendar(month, year);
   }
}
function setNextYear() {
var year = document.calControl.year.value;
if (isFourDigitYear(year)) {
var day = 0;
var month = document.calControl.month.selectedIndex;
year++;
document.calControl.year.value = year;
displayCalendar(month, year);
   }
}
function displayCalendar(month, year) {       
month = parseInt(month);
year = parseInt(year);
var i = 0;
var days = getDaysInMonth(month+1,year);
var firstOfMonth = new Date (year, month, 1);
var startingPos = firstOfMonth.getDay();
days += startingPos;
document.calButtons.calPage.value  =   " Su Mo Tu We Th Fr Sa";
document.calButtons.calPage.value += "\n --------------------";
for (i = 0; i < startingPos; i++) {
if ( i%7 == 0 ) document.calButtons.calPage.value += "\n ";
document.calButtons.calPage.value += "   ";
}
for (i = startingPos; i < days; i++) {
if ( i%7 == 0 ) document.calButtons.calPage.value += "\n ";
if (i-startingPos+1 < 10)
document.calButtons.calPage.value += "0";
document.calButtons.calPage.value += i-startingPos+1;
document.calButtons.calPage.value += " ";
}
for (i=days; i<42; i++)  {
if ( i%7 == 0 ) document.calButtons.calPage.value += "\n ";
document.calButtons.calPage.value += "   ";
}
document.calControl.Go.focus();
}
function getDaysInMonth(month,year)  {
var days;
if (month==1 || month==3 || month==5 || month==7 || month==8 || month==10 || month==12)  days=31;
else if (month==4 || month==6 || month==9 || month==11) days=30;
else if (month==2)  {
if (isLeapYear(year)) { days=29; }
else { days=28; }
}
return (days);
}
function isLeapYear (Year) {
if (((Year % 4)==0) && ((Year % 100)!=0) || ((Year % 400)==0)) {
return (true);
} else { return (false); }
}
// End -->
</SCRIPT>

<big>Select-A-Month</big>
<FORM NAME="calControl" onSubmit="return false;" method="post">
<TABLE CELLPADDING=0 CELLSPACING=0 BORDER=0>
<TR><TD COLSPAN=7>
<SELECT NAME="month" onChange="selectDate()">
<OPTION>January</OPTION>
<OPTION>February</OPTION>
<OPTION>March</OPTION>
<OPTION>April</OPTION>
<OPTION>May</OPTION>
<OPTION>June</OPTION>
<OPTION>July</OPTION>

<OPTION>August</OPTION>
<OPTION>September</OPTION>
<OPTION>October</OPTION>
<OPTION>November</OPTION>
<OPTION>December</OPTION>
</SELECT>
<INPUT NAME="year" TYPE=TEXT SIZE=4 MAXLENGTH=4>
<INPUT TYPE="button" NAME="Go" value="Build!" onClick="selectDate()">
</TD>
</TR>
</FORM>
<FORM NAME="calButtons" method="post">
<TR><TD><textarea FONT="Courier" NAME="calPage" WRAP=no ROWS=8 COLS=22></textarea></TD><TR><TD>
<INPUT TYPE=BUTTON NAME="previousYear" VALUE=" <<  "  onClick="setPreviousYear()">
<INPUT TYPE=BUTTON NAME="previousYear" VALUE="  <  "  onClick="setPreviousMonth()">
<INPUT TYPE=BUTTON NAME="previousYear" VALUE="Today"  onClick="setToday()">
<INPUT TYPE=BUTTON NAME="previousYear" VALUE="  >  "  onClick="setNextMonth()">
<INPUT TYPE=BUTTON NAME="previousYear" VALUE="  >> "  onClick="setNextYear()">
</TD></TR>
</TABLE></FORM></FONT>

<p align="center">This free script provided by<br />
<a href="http://javascriptkit.com">JavaScript Kit</a></p>