Paid 2018 property taxes via check on 12-26-18; check was cashed on 1-6-2019. County receipt says: Effective Date of 12-31-18 AND date processed of 1-6-19.   Put in '18 tax or '19?

Texas (can pay 2018 taxes thru Jan 2019), so locally could have done for either year.  The crossover of years is what is confusing me (had I sent on 12/1 and processed 12/10, it would clearly be last yr.  likewise if I had sent 1/2 and processed 1/10, it would be current tax year).  

Which does IRS count? Processed date or effective date? I presume the former?