This is called Comic-Book Time aka Floating Timeline aka Sliding Timescale.
The problem is this. On one hand, Superman is a high-selling,
successful character with a lot of licenses and so on based off of
him. You don't want him to age or die, because that means losing that
successful character. On the other hand, Superman exists as part of a
greater universe, and if all the stories in that universe are
continuously frozen in time, that cuts off a lot of possibilities. So
what do you do? Comic-Book Time. You use the illusion of time passing.
You never refer to specific dates if you can help it, and you let
characters change, but only a little.
Also related to Not Allowed to Grow Up
Or you might be thinking of Negative Continuity
Not only is there no established continuity, but the show is free to
completely wreck the continuity and be assured of a full reboot by the
start of the next episode. Burned a hole in your favorite outfit?
Don't worry, it'll be better next episode. Burned down your house? No
worries, it will be back next time. Turned into a frog, died,
destroyed the universe? No problem! If one episode ever continues from
the last, it's only because it's part of a storyline too long for just
one episode — don't expect any apparent changes from the previous
episode to be recognized outside that specific storyline.