If you save the document with a Time and Date format, every time you open the document, the date and time will change automatically. Read: How to Mark a Document as Final in Word.

Learn how to change Undo limit in Microsoft Word. You can increase or decrease the number of undo limit using Registry Editor.

Microsoft Word has a date field, but it's not easy to use in an expression. If you need to update dates in a document, use Excel to calculate the dates and then run a mail merge.