Windows 7 doesn't show AM/PM symbols in system tray clock. Also it shows the time in 24 hour format i.e. 18:03
If you want to get back the AM / PM symbols, then this tutorial will help you.
1. Type intl.cpl in RUN or start menu search box and press Enter. It'll open Regional and Language Options window.
2. Now click on "Addition Settings" button.
3. Go to "Time" tab and you'll see the value of "Long time" will be set to HH:mm:ss. Just add tt at last of the value, i.e. change the value to HH:mm:ss tt
4. If you want to show the time in 12 hour format (06:30 instead of 18:30), then change the value to hh:mm:ss tt
5. That's it. Click on Apply button and you'll get the AM / PM symbols back in system tray clock.
PS: If you want to customize AM / PM symbols to something else, then follow this tutorial:
NOTE: If you want to hide Time in system tray, put a blank space in "Long time" text box under "Time" tab.