I assume that as you posted the Siemens help files for doing this that you haven't actually done anything in the PLC which was what Jean Pierre was refering to.
I will give you some clues as to what to do.
In OB1, in the temporary data, there is the OB1_DATE_TIME, you can use this instead of the system date and time. (it's the same thing really). This data is 8 bytes long, so you could use SFC20 (block move) to move this local data to a DB area that is also 8 bytes long. Set the DB up in bytes and label them YEAR, MONTH, DAY, HOUR, MINUTE, SECOND, MILLISECOND and DAY_OF_WEEK. Although, to get the day of the week you need to some manipulation with the bytes.
This thread explains this
http://www.plctalk.net/qanda/showthread.php?t=9284
Then in the TP170 set up some output fields that point to the correct area (byte) in the DB, voila you have your system time displayed on the TP.
You could also search this site for the keyword 'SFC1', you should get a good few hits that would help you in your quest.
Then, if you still can't get it working, post what you have and we will see what we can do.
Paul