If you press F5 in notepad, it will insert current date and time in the file.
If you would like Notepad to insert the date and time every time you open a particular file all you need to do is add a .LOG in the first line of your text file and close it. Whenever you open the file with that text in the first line in Notepad, it will insert the current date and time at the end of the file.
Isn't that really cool!!