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I'm not sure about the date thing. When we bought one of our houses in 2014, my wife's deferred student loans counted towards our DTI.
Here is Shane's thread about it...
Is your wife's income needed to get approved for the loan? I mean, is there anyway you can buy the house without her being on the loan? I'm pretty sure you can add her later. Just a thought.
Someone pointed out a link to Shane's post about the changes made back in September and unfortunately there is very little getting around adding at least 2% of a student loan's balance to the D2I of a person applying.