By default when you start using your Windows 8 Lumia 520 phone the date and time gets set automatically. You might only want to change the clock settings like 24 hour or 12 hour. Here are some steps on how to do that.
From the home screen, swipe towards the left where you get all the icons. Drill down and select Settings. From the system application setting you can find the date+time. Select date+time, here switch off the 24-hour clock, by default this will be on.
When you want to change the date, then switch off the set automatically button. You get below options to change the date and time.