This is so simple to do for anyone..you need to try it by yourself...Do you google it?If you expect code from here then it willn't increase your coding skill.Ok,then lets see..
DateTime.Now.ToString() will show you current time.You can use different foramts for that..suppose if you use DateTime.Now.ToString("hh:mm:ss")then it will display time as hour,minute,second format.If you use DateTime.Now.ToString("hh:mm:ss tt") then it will represent whether it is AM or PM.There are lot of articles in MSDN for it..see those..here i give you an simple clock example code for reference that you can understand easily how to build your application..
http://www.c-sharpcorner.com/UploadFile/0524d6/digital-clock-of-windows-7-phone/[
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http://mobile.tutsplus.com/tutorials/windows/build-a-windows-phone-7-clock-app/[
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