I did some testing myself, and there were plenty of others who tried all sorts of permutations and combinations of times, dates, and places. Nobody I ever heard of found any effects, and I eventually came to the comclusion that that there was a ceiling of about level 3 on the permitted enchantments. Below that, I thought that what you got was pretty well random.
Getting that Titanic Trident of the Dragon blew my conclusion right out of the water, though, and I'm now working with the tentative hypothesis that Enchant Item gives goodies according to some abstruse probability calculation. This will be hard-coded into the game engine, and I don't have the skills to work out what it is.