I hereby certify that XXX,a student __1__ in XXX at XXX University,__2__ __3__our company from June 15,2011 to September

I hereby certify that XXX,a student majored in XXX at XXX University,had worked in our company from June 15,2011 to September 5,2011 and January 18,2012 to March 3,2012.如果能解释一下其他几项为何不对，就更加万分感谢了！！证明的事实是 “过去，曾经有个某某学生在某某学校某某专业班级就读"得用过去时，而from June 15,2011 to September 5,2011 and January 18, 2012 to March 3, 2012只是过去的一个时间段，用过去完成时即had+过去分词的结构。如果去掉时间段，是不是可以用She (has completed/fulfilled) an internship in this company?对呀，去掉时间段就属于过去时的范畴了呀。。。