Where does the bible say 'Don't set dates' as some say it does?

The bible is VERY important with numbers, they represent many things, including dates which represent a lot as well. The bible doesn't tell you not to set dates. The only thing i remember, is that Jesus said no man knows the day the Son will return, not even the son, only the father, so if anyone tells you "God's coming back this day" you know that they aren't exactly following scripture.