You can only claim a deduction in the year that you actually make the payment. Since it's already January 1, you don't have any flexibility to delay
2018 deductions. Anything you already paid in 2018 can only be deducted
on your 2018 tax return. You can't just choose to wait and claim the
deduction for 2019.
The only way to delay a deduction is to delay making the payment. For example, you could delay a deduction for contributions to charity by making the contribution in January instead of December. If you normally make a state estimated tax payment in December, you could delay it until January (before January 15). But other types of deductions, such as real estate tax or mortgage interest might not be as easy to delay. Your town or your mortgage lender might not be very understanding if you pay late, and they might charge a penalty or additional interest.
