Feivel, I suppose technically “I am a liar” could work, but it doesn’t fit as neatly as the other answer. If you subject all answers to the test of “its truth value cannot possibly be determined” then you essentially create a whole host of potential answers. Why couldn’t the soldier have said “My father’s name is Daniel?” The enemy has no idea what his father’s name is, so he can’t evaluate the truth of the statement. Perhaps this is a weakness in the riddle itself. (I didn’t write it; different versions can be found online.)
Squeak, I don’t think the answer could be 27. That’s why I originally said 31, because I was assuming that the snail climbed 1 foot a day, but on the 30th day he still would not be “out of the hole” yet; it would take him another day to get onto solid ground. If he climbs 3 feet a day, that means he climbs 3 feet a day and no more, not even the few inches that he would need in order to climb over the lip.