The more I think about it, the more "Day One" comes across as nonsensical.

I woke up this morning, we had multiple data centers across the globe filled with millions of servers... Day One.

I clocked into the swing shift, we have bins overflowing with unthinkable amounts of returned items, many of which are semi-perishable toiletries and personal hygiene products... Day One.

As a simple-minded catchphrase to orient one's thinking, it becomes more and more absurd as the company scales. But Jeff don't care, he punched out a long time ago.