How to ask employer to write in the offer letter actual dates I will be working (Feb 2021 - sept 2021) instead of what they wrote (Feb 2021 - april 2021)?

My new employer said during the interview and phone call that I will be working until September 2021. When they emailed me the offer letter, they told me to ignore the end date of April 2021 since "it will be extended until September upon hire." Additionally they said, "Again, we are constantly applying for more funding and we are confident we will be able to offer another extension and higher position with the new fiscal year (October 1st)."

I believe the reason they are putting the end date as April on the contract instead of September is for me NOT to receive the organization's benefits program (which you receive if you work there for over 6 months).

Plan on calling the employer soon to make sure the end date is stated as September, but idk how.
How do you recommend asking for this in a professional, polite, yet firm manner?

Thanks!

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