You can put timing points with a negative offset by typing it in or by using trickeries. In the editor, go to the "Design" tab and scroll to your desired time, and place a timing point by going to the timing editor thing and clicking "Use Current Time."
I use this (in my offline beatmaps I'm working on) to get the big beats to land properly when the song starts immediately.
(Imma have some fun, use your best southern accent)
You cannot physically put anything at a negative offset/time, and also there is no reason for it as the song hasn't even started. If you want to, you could edit the mp3.
Back to the point, YOU CANNOT HAVE NEGATIVE TIME. Your science teacher will end you if you have an answer with negative time. However, I'm sure your math teacher will accept your answer, even if you are breaking the very laws of feeesiiics