"Last date in 2018 to make a distribution from a trust that will count for 2017"
In order to count for the previous tax year, the distribution must be made within 65 days after the close of that tax year.
See Treas. Reg. §1.663(b)-1
The last date in 2018 that would have counted as a distribution in 2017 was March 6, 2018.