“The oldest and strongest emotion of mankind is fear, and the oldest and strongest kind of fear is fear of the unknown.” -H.P. Lovecraft
There’s a unique relationship between everything that exists in the Universe today — the stars and galaxies, the large-scale structure, the leftover glow from the Big Bang, the expansion rate, etc. — and the amount of time that’s passed since it all began. When it comes to our Universe, there really was a day without a yesterday, but how do we know exactly how much time has passed between then and now?
There are two ways: one complex and one simple. The complex way is to determine all the matter and energy components making up the Universe, to measure how the Universe has expanded over the entirety of its cosmic history, and then, in the context of the Big Bang, to deduce how old the Universe must be. The other is to understand stars, measure them, and determine how old the oldest ones are.