[HansJ]

Add a set of curly braces around the code you want in the function e.g.

var a =1;
function displayDate()
{
	a=a+2;
	if (a==1)
	{
		document.getElementById("demo").innerHTML="Hello";
	}
	else
	{
	document.getElementById("demo").innerHTML="GoodBye ";
	}
}

The if statement that changes the innerHTML was not being called when you clicked the button, only the addition part

Make sense?