If you want the date in your document to update each time the document is opened,

check the Update Automatically option in the Date And Time dialog box.

Explanation:

Assuming that you are taking about a document in Word (not Excel), when you want the date in your document to update each time the document is opened, check the Update Automatically option in the Date And Time dialog box.

To do that do this:

1) Click on the place of the document on which you want the date to be displayed.

2) Go to the Insert tab on the main bar.

3) Open the Text group

4) Click on Date & Time

5) Select the format you want

6) Check the box Update automatically

The answer is length!