Usage of before it's too late

Let’s save the Earth before it’s too late!
We should do something before it’s too late!
Tell her before it's too late.

before it’s too late is a fixed sentence or what does it refer to? It here functions as an impersonal subject like It is hot, It is 2 o'clock, etc or it refers to the whole sentence ahead?

What do you native English speakers think?

Thank you so much as usual in advance.