According to IRS Instructions for Form 8863 for 2017, "[q]ualified education expenses paid in 2017 for an academic period that begins in the first three months of 2018 can be used in figuring an education credit for 2017 only". According to the same publication, "academic period" is defined as payments made in the calendar year for academic periods beginning in the calendar year or in the first 3 months of the following year. Payment made in 2015 cannot be used in 2016.
