If matter was not created out of nowhere then something that once had a certain percent of Uranium had to exist before it had that certain percent so if you assumed it started 100% Uranium and turned to lead then you could guess a date but if it was 100% Uranium then it must have been something before it was 100% Uranium and something before that and so on
Also you could start at some other % Uranium
It is very arbitrary because you do not know the starting point
@TrevorGoodchild @pepsi_man
For example an object could have had some lead in it that was already there and did not come from Uranium as it's starting point
You can not really know how much lead it started with and how much Uranium unless you know what it was like in the initial condition it was formed
And if you witnessed it in the initial condition it was formed then you would already know how old it was from when it was formed and not need a dating method to date it