Her father's birthday had been in June.怎么翻译

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Her father's birthday had been in June.怎么翻译
Her father's birthday had been in June.这句话的意思好像应是“她父亲的生日在六月.”那为什么要用had been? 这我很不理解.还有, 这句话是什么时态?请教朋友们,谢谢!

她爸爸的生日已经在六月份过了.
had been in 可以是一些短暂动词过去完成时的代替,具体哪些,我不记得了.