_It__ _is___ two year _since___ they joined the swimming club.
翻译：自从他们加入游泳俱乐部已经两年时间了.
语法知识：
It is（was/has been)...since...,时间计算一律从since从句的动作完成或状态结束时算起,如：
It's three years since he lived here.
他不在这儿住有三年了.
It's three years since I smoked.
我不吸烟有三年了.
It’s three years sinceI began to smoke.
我吸烟有三年了.