Though only five letters,
From my start to my end,
I can have the meaning,
Of "Gone out with a friend."
Add two letters of negation
If you follow this rhyme
And my meaning is now
"Not classified according to time."
Add two more letters in front,
Again that mean "Not"
Now my meaning is "flooded."
So the answer is what?
Original five letter word=> DATED
Adding two letters of negation in terms of 2nd stanza: UN+DATED => UNDATED, which means "Not classified according to time."
Again adding two letters in front (prefix) which is a synonym of "not", in terms of 3rd stanzs: IN+UNDATED=> INUNDATED, which means "flooded".
So the answer is "DATED"