Singular or plural?

The past six months [color=red]have been awful for her.

Why “have” is used here instead of “has” ? When do we consider a period of time a unit?

Thanks.

It is up to the speaker to consider it a unit or a number of units. ‘Has’ is also possible in that sentence.

in the above sentence" have" used as main verb not as auxiliary
so as main verb , “have” does not change to has in case of thired person singular number

That is the strangest grammar I have ever heard, Ashique. Who taught you that?

i read somewhere, if i am wrong please clarify it, Mister Micawber

I’ll clarify it. This is completely wrong:

‘Have’ always changes to ‘have’ in 3rd person singular.