On a failed 1031 exchange, capital gains are paid in which year?

[Michelle R.]

We have a property sold end of December 2017. If we were to not procure a replacement property by the 180 day mark in June 2018, there would be capital gains owing. Would we pay the capital gains for tax year 2017 or for tax year 2018? Asking because the tax deadline for 2017 is next week and wondering if this can be put off til 2018. Thank you in advance for any help.