"select today-1 units month from systables", run this.

i've been using this "today-1 units month" for so many applications and not only me, i've seen it in many applications here in my company, now this date computation is giving an error just for this day only, May 31, maybe some other days to like Dec 31, March 30.

my question is, is this a logical error or do we have to update something on the informix? am i the oly one getting this?