If this is what you wish to achieve:
A single modification to one of Messenger's language file is all it takes. There may be another method of which I am unaware, however the method I used is as follows:
Download a program such as
ResHacker (free).
Now in ResHacker you need to open up the language file for Messenger. This is called "mslang.<version number here>.dll" and can be found in "C:\Program Files\Windows Live\Messenger" (obviously this directory may be slightly different depending on the setup of your computer).
Then, expand "String Table" on the left and find "138". Expand this and select it's child ("1033" in my screenshot). Then on the right, find the line with "%1 writes:" and change it to "%1 writes (%3):".
Compile the script, then File>Save (Make sure Messenger is closed fully first!) to save the changes. There should now be a timestamp next time you send/receive a written message.
Hope this helps