If the start date decision is already decided... then here is what I think the logic on choosing it was.
These are the start dates used in previous Europa Universalis games: 1399, 1419, 1444, 1453 and 1492. I think that it will be a new start date, and I would applaud this decision. Although I would love 1415, I think it is way too close to 1419, so I'll have to rule it out.
Regarding the end date, 1789, 1801, 1820 and 1821 (twice) have been used. I believe the Napoleonic Wars are a must, so either they stick to 1821, or perhaps change it to 1815 (with potential plans for a new game between 1815 and 1836) or 1836 (to link up with Victoria). Regardless, it won't make much of a difference to decide on the start date.
This leaves us with three choices:
#1 - before 1399
#2 - between 1419 and 1444
#3 - between 1453 and 1492
Of these, I feel like #3 is the least likely simply because Byzantium does not exist there, and the devs would never hear the end of it. If i were to pick a date here, perhaps 1469 when Ferdinand II of Aragon and Isabella I of Castile married.
I am also hesitant in picking #1 because the game would drag on a bit. Although I would pick 1486, when the Treaty of Windsor was signed.
So that leaves us with #2, and I would go with 26th of March of 1431 - The Trial of Joan of Arc.