This is the javascript you need to add to the LockBackground.html page if you want to get custom time, day and date display.
But in order to not have a double time and date displayed you have to disable the time and date that comes with iOS. To do that go to Cydia and search for "Lockscreen Clock Hide", its free in Sections->Tweaks (also install Lockscreen Dim Delay, which lets you set a different time interval before your Lockscreen turns black).
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