DAP should be just fine, we stored it in a overhead feed bin for 10 years. Bin is vented. Would normally get some in August or September use in October then get more in December and use in March and April. Sometimes there would be a thin crust on top or some hard fertilizer on top of the door and some hard fertilizer along the wall and slope from the sight gauge leaking. 

Stored some thru the summer of 19 it came out just fine in the fall.  That was the summer that I got tired of reading on here about how everyone was so terrible wet when most of the complainers had significantly less rain then we had gotten. We now store DAP in a 125 cone bottom bin. Fines will cause it to not flow out very well, in the 12 years I have been storing it, that has been a problem twice. Length of time in storage matters some with how well it flows. It will flow out better if you take some out everyday verse once every 2 weeks, but can not tell the difference between 2 weeks and 2 months. 

Question for those that say it will not work: What have you seen that says it will not work? Lots of nos without stating any past experiences.