The phrase "not before" would include the time period after 2031. It could happen any time but "not before" 2031. The phrase "not until" dose not include the time period after 2031. It will not happen until 2031. This means it will not happen before or after 2031 but during the year 2031. Hope this helps. I can see how that could be tricky.

Because 'Not Until' is treated as a time expression which puts emphasis on something that doesn't occur before a certain point in time or before something else has happened, which introduces a condition. Example A: Mum I am going out to see a friend. and play now? B: Not until you've washed the dishes. I hope that helps.