I think most revolutions would be difficult to pick a single start date for. For whatever reason, the US’s education system has gone all in on the Declaration. We have a lot of mythology surrounding our founding documents and founding fathers, so what these signs are really saying is, “we are carrying the same torch of freedom that our founders carried”. It’s symbolic of their willingness to die for their cause (or more likely, for other people to die for their cause).

As some bonus context, this was exactly what the south thought they were doing during the American Civil War. They thought the tyranny of having taxes levied on your tea was exactly equivalent to not being allowed to own slaves. And they called the civil war the second american revolution for a while.

Yeah, it is kinda weird, but here in the USA, it’s exceedingly common to see 1776 on bumper stickers or flags of Hardline reactionaries. I guess to show that they are real American patriots

It’s a religious thing.