Hello @alsgpwns, welcome to the GitHub Support Community!

Since you’re incrementing and decrementing numbers, I would recommend using the -- and ++ operators. This way the changes made to mm will be stored temporarily in memory and allow you to move forward or backward.

Here is the updated code (see the IF statements at the top of each function):

function onLeftClickTest() {
    if (mm != 0) mm--; // decrement mm if NOT 0 (January)
    firstdayInst = new Date(yyyy, mm, 1); 
    getfirstday = firstdayInst.getDay(); 
    lastdayInst = new Date(yyyy,mm,0);
    lastday = lastdayInst.getDate();
    showMonth(mm);
    showDay();
    showDate();
document.getElementById('CalendarBody').innerHTML=text+
 `
<button id="PreMonth" type="button" value="click" onclick="onRightClickTest();">></button>
<button id="LastMonth" type="button" value="click" onclick="onLeftClickTest();"><</button>`;

}
function onRightClickTest() {

if (mm != 11) mm++; // increment mm if NOT 11 (December)

firstdayInst = new Date(yyyy, mm, 1);

getfirstday = firstdayInst.getDay();

lastdayInst = new Date(yyyy,mm,0);

lastday = lastdayInst.getDate();

showMonth(mm);

showDay();

showDate();
document.getElementById('CalendarBody').innerHTML=text+
 `
<button id="PreMonth" type="button" value="click" onclick="onRightClickTest();">></button>
<button id="LastMonth" type="button" value="click" onclick="onLeftClickTest();"><</button>`;

}

If you want to reset the month when you get to the end, you can add an ELSE statement, like the one below.

else {
  mm = 11 || 0; // 11 for onRightClick OR 0 for onLeftClick
}

This’ll also let you increment or decrement other variables, such as the year.