You can add your name or anything you like that consists of 8 characters or less. This will replace the AM or PM next to the system time. But you can corrupt some trial licenses of software that you may have downloaded.
1. Open RegEdit
2. Go to HKEY_CURRENT_USER\Control Panel\International
3. Add two new String values, "s1159" and "s2359"
4. Right click the new value name and modify. Enter anything you like up to 8 characters.
If you enter two different values when modifying, you can have the system tray display the two different values in the AM and PM.