Conception
(A)    Conception requires you to have the entire invention in your mind and have use in mind to satisfy utility. So if 2 people are fighting over a patent, date of conception is when you have a specific concrete use satisfying the utility requirement.
(B)    Why? When choosing between two patent holders, need to decide what the touchstone is. Part of what we want is person to come up with beginnings of how to use it, that is the person who should be rewarded when 2 people are competing for the same patent.