1.it✀s +时间+since

It's been I-don't-remember-how-long since the last time I cared about all these bullshit...

dude, you need to calm down. you know need to know the rules, as long as you can communicate with human beings you're ok.

号楼niobium

因为它们各有自己的句型。如下
it has been/is +一段时间+ since且句子用一般过去时。
it was/wasn't + 一段时间 +before 从句用一般过去时
it will be/won't be +一段时间 +before 从句用一般现在时表示将来
所以在选择用before还是用since只要记住它们的不同点就可以了。即它们的主句的时态不同。相同点是它们都用在一段时间之后。如下面这些例子
It is two years since I came here.自从我来这到现在两年了。
It was two years before he came back.过了两年他才回来。
It will be two years before he comes back.要过两年他才能回来。
It wasn't two hours before he came back.不到两个小时他就回来了。
it won't be two years before we meet again.不用两年我们就能再见面了。
注意since从句中用延续性动词时表示从这个动作结束时算起。如
It is two years since I worked here.我不在这工作两年了。
从句用非延续性动词表示动作从开始算起，如
It is two years since I came here.我来这两年了。
.It's years ( )i enjoyed myself so much 这句中动词enjoy就是一延续性动词。所以填since .译为“我好多年都没这么高兴了”即从动作结束时算起。