The question to ask yourself is "Am I making this IRA contribution for my future or and I making this IRA deduction for a tax deduction?" If you were already going to make a contribution for 2018, then do it by the 16th. If you want a possible deduction, then make the contribution anytime during 2019.
Since you aren't getting a deduction for your contribution, have you considered opening a Roth IRA for tax-paid donations?
