A div is just a container that CSS uses to format the layout of a webpage.
Without seeing an example of the layout it is incredibly hard to distinguish what exactly is needed.
Personally I think you would be better having just one div for the chat and a then 2 radio buttons for turning time on or off.
You would need to read which radio button is ticked, then use that status to add the timestamp to the start of the actual chat sentence.
'Time off' would result in "this is a chat string", but 'time on' would result in "[TIME]This is a chat string"
That way there's no need to worry about formatting the time div.
However, if you must, then you could see how many characters in a chat string makes 1 line, then you'd have to divide the characters in the string by the line length to see how many lines you have. Then add a \n for each line in order to keep it equal.
If that makes sense