I seem to hear/read different versions of this phrase but am not sure what is/are
the correct one(s)

Which of the following is/are the correct usage?

"I am in a time crunch"
"I am on a time crunch"
"I am at a time crunch"
"I am in a crunch time"
"I am on a crunch time"
"I am at a crunch time"

Can any native speaker help? Thanks a lot.
Or how do we use this phrase correctly?
crunch time ~ the period of time just preceding a deadline when everything must fall into place; a time of tension for the participants.

The teacher wants the essay tomorrow. It's crunch time (for me)!

time crunch ~ the condition of having too many things to do and not enough time to do them all.

Sorry I can't stop and help you; I'm in a time crunch with all my projects.

I'm at crunch time (informal).

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 CalifJim's reply was promoted to an answer.
I am on a crunch time
anonymous I am on a crunch time

That does not make sense.

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