Take everything between your script tags and put it in another file. You should save this file with a .js file extension. Let's pretend you save it as textChange.js.
Now the simplest thing to do would be to include the script file just after your <div> tag -- so basically where the <script> tags and code were before, write:
<script type="text/javascript" src="textChange.js"></script>
This assumes that 'textChange.js' is in the same folder as your HTML file.
...
However, that would far too easy! It is generally best practice to place <script> tags in the <head> of your HTML file. You can move the line above up into the head but then the script will load before your <div> does--it will try to do what it does and it will fail because it can't find the div. So you need to put something around the code in your script file so that it only executes when the document is ready.
The simplest way to do this (and there may be better ways) is write the following...
window.onload = function () {
var d = new Date();
var funnyDate = (d.getFullYear() + "" + (d.getMonth()+11) + "" + (d.getDate()+10));
if ((funnyDate>=20131916) && (funnyDate<=20131923))
{
document.getElementById("textChange").style.display ="block";
}
}
This will mean your script is in the head where it should be and that it only performs when your whole page is ready, including the div that you want to act on.
Hope this helps.