Here's an MSDN article that explains a simple trick to create automatically timestamped entries in a text-based log file. I use Textpad instead of Notepad, and the technique works with Textpad as well. The basic idea is that you open a file in a text editor such as Notepad or Textpad, then put ".LOG" on the first line. Close the file. Each time you reopen it, the current date/time will be appended to the file.
I realize that this is an "old trick" for some, but maybe others haven't heard of it.