If the 1098-T shows an amount billed in December 2017, but the classes did not start, AND WERE NOT PAID FOR until 2018, you may report that expense next year. Otherwise the 1098-T is reported in the year it is issued. Education credits are based on what you paid in the year for education expenses of that year and/or the first 3 months of the following year. You can't save up expenses paid and claim in a different year. 

CLICK HERE for IRS Pub 970 Education Credits

CLICK HERE for TurboTax information on Education Credits