when you start your window, you see the time with AM and PM symbols.
Here you can show your text as These symbols rather then AM and PM Symbols.
For this Follow :
First Click on the time shown on the window task bar.
then click on Change time and date settings.
then click on change date and time
After that click on change calendar settings then a window of customize format
now choose Time Tab and go to AM and PM symbol field and type your text. now click on apply and OK on every window.
Thus you have done it.