Emcee, I'm not sure that is any special significance about TIME in the title. If there is, it escapes me. I wouldn't worry about it.
Phil535, I think there are eight possible triplets, not four, though there are only four that will work for the puzzle. Two of the unused set use 25 as the two-digit square and two use 81.
Orson, re the letters used, my guess is that the setter wanted to span A-Z. GHI might not be suitable because of possible confusion of I with 1, and MNO because of possible confusion of O with 0.
As for difficulty, all that is needed to get started is list of squares. It doesn't take that long to scan and select the twenty-one suitable 4-digit squares to add to the six 2-digit squares and the thirteen 3-digit ones. If 16, for example, is selected as a provisional 2-digit square, there are only two possible 4-digit square to go with it because all the others have a 1 or 6 in them.