It’s a technicality but this is just a figure of speed for a date, not a religious reference.
And I’m pretty sure the separation of church and state is just that separation of the two. Which has gotten long lost with one being a favorite who’s is allowed to play double standards and influence the government as it pleases to get it’s perks back.
As Gundam posted quptes, many of the forefathers wanted nothing to do with religion in the new land.