Understanding dates in Plateau 6.2

I'm really hoping someone can help me understand how the dates work in Plateau 6.2.  How do I get the items to show the correct due dates?  The only way I've seen so far is to change the required date of each item within each employee's learning plan.  But we have 752 employees.  There's got to be a better way.

When we initially set up the system.  We programed each item using the document's effective date as the "revised date".  Then we added a 90 days as the Initial Period on the Design Data tab.  So when we assigned the curriculum, it cause all items to have future due dates, even if the effective date was 2 years ago!  Now I don't know how to fix it.  I don't know how new items should be set up in the future.  I don't know how to use the revised dates.  I don't know what Basis Date means.  I don't know what Threashold means.  I'm lost! 

Can anyone help me?

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