How do I create variable data (today's date) that updates each time the document is opened?

Example Acrobat JavaScripts has the code and example PDFs. The description of how to add the code is outdated but the examples continue to work and are accessible through Prepare Forms => More => Document JavaScripts or through the JavaScript tool.

Add a text field to the PDF. Let's say the name of this text field is "CurrentDate". Add this code to a Document Level Script

this.getField("CurrentDate").value = util.printd("mm/dd/yyyy", new Date());

Here's an article on document level scripts:

Basic Document Level (Document Open) Scripts