父母来荷探亲：180天内最多停留90天的疑问。。。

My mother got an 1 year multi entry visa to Schengen countries, valid from 2-Feb-2013 to 31-Jan-2014.

Visa says she can stay in Schengen countries maximum 90 days per 180 days.

She entered Netherlands on 2-Feb-2013, and returned to China on 6-Apr-2013. She plans to visit Netherlands again on 6-Jul-2013.

Question: this time until when she can stay maximum in Netherlands?

其实问题就是：我妈第一次入境的180天内没有呆满90天就回去了，然后第二次再来（入境日期还在这180天内）能把第一次没呆满的天数呆完再继续算第二个180天么？