It will be rejected by mempool,
I tried to broadcast transactions before a valid date and got this error "the transaction was rejected by network rules. non-final"
This error above was the same in both cases of
Block height and
Date and Time.
I read that you can broadcast it 2 hours before it becomes valid (but never tested this).
I have set parameters today for hour 23:00, I'll try to broadcast the transaction around 21:10 and see if it's possible.