And has, as you may guess, a rather random element to it. The sort of things that makes you contemplate a D&D wand of wonder.
Except of course it works much better in a fictional setting. The author doesn't have to roll something up. Even if he does, wanting to shake up the possibilities, he can smooth it into the tale with appropriate reactions. There's no danger that the main character will actually come up with a silly scheme that will inspire the author to say, Rocks fall, everyone dies.
Now, when they are trying to figure out what it does, Wildcard is perfectly capable of announcing that, at the tone, it will be 4:23 am -- which is wrong* -- but in actual field usage, it always produces something that (with suitable hindsight) is useful for the situation. An author who wanted the sword to work the other way could make it always produce something that does not work for the sword's owner, for whatever reason, perhaps with hindsight. (Preferably with hindsight. One of the things I disliked about Mercedes Lackey's sword Need is that its effect always was immediately and obviously useful, and only once (in the stories I read) was it even a surprise to the person holding the sword.)
Ah, the differences that genre makes.
*Now the muse is contemplating a magical item that announces the time wrong except that if you really want to activate it, you need to go to the place where it's the right time. Helps to notice it's always off by the same amount in the same place.