Schengen 90 days: what's counted?

I'm a Chinese citizen and have a Spain long stay study visa (type D, multi-entry) that's valid for 180 days. The visa expires in April, and I want to know if it is possible to immediately apply for a short term Schengen visa that's valid from May? I've heard that the 90 days per 180-day-period only applies to short stay visas, and the time spent in Spain under the type D long stay visa is not counted. Is that true?