Begin your text with a first line exactly like this:
.LOG
Then save the document. Every time you open a file which begins with this line, tpad will automatically insert the locale specific current date and time to the end of the text (this is the same as manually choosing Date/Time from the Edit menu).
You can also use this function to write your diary with tpad!
By default this facility is turned on. If you don't want it, just set the variabile tpad(logfac) to no in tpad configuration file.