The alternative base period is the last four completed quarters at the time the claim is filed. Right now and for the rest of this month we'll be in the second quarter (Apr, May, June), so the alternate base period would be April 2015 - March 2016.

It's important to note that using the alternate base period is not a choice, and can only be used when you don't meet the eligibility requirements using the standard base period. 

Source: http://www.detr.state.nv.us/ESD%20Pages/ESD_docs/UI_Claimants_Handbook.pdf