For 8615 purposes, my question: does the child have to be a full-time student the entire tax year, at the end of the tax year, or just at some period during the year?

In the instructions for 8615, it describes someone having to use the form if a full-time student and under age 24 at the end of the year.  Does full-time mean for the entire year, or just one day in at least 5 calendar months of the year?