He was been in the army for five years 的同义句

问题描述:

He was been in the army for five years 的同义句
____ five years____he____the army

It's,since,joined为什吗啊.........................(It's ) five years(since ) he(joined )the army 的意思是“自从他参军,到现在已经五年了。”和原来的句子意思一样前面为什么用It's?It's即It is,表示“时间是。。。”的意思