When do taxes become back taxes?

[Skye Payne]

Hey y'all! Very excited to start my first post/career/investment life with BP!

So I'm working on a potential first deal. Sent her a contract, she countered by saying that because the offer is so low, the back taxes should be included. I looked in my contract, and I didn't have verbage specifically stating what to do with back taxes, only that taxes will be prorated between seller and buyer.

So I'm slightly put off by this, and I'm considering adding in the back taxes because it's not really that much and I think there's enough room number wise. My question is, how does current year tax work? When is it considered back tax? The tax bill says that it's due on September 1st, pay by January 5th, and intrest starts to accrue on January 6th. Could I pay the previous tax year, and then prorate this year? Thanks for all your help in advance!