The time when the U (uranium) formed in super nova process is not relevant. In fact it is taken into account explicitly when you make an isochron plot. The idea of radiometric dating is the following:

Let us say we have a silicate melt and in the melt there is some Rb (rubidium) (I am going to use the Rb goes to Sr (strontium) as the example, since U has three isotopes of Pb (lead), and it is more complicated, although the principle is the SAME).

Now from this liquid a crystal of feldspar grows and a crystal of biotite also grows. We want to date the time when these crystals grew in the magma. This gives us the age of the granite rock that forms from crystallization of the melt.

This crystal will incorporate some Rb 87 into its solid lattice. The crystals will incorporate some Sr 87 from decay of Rb BEFORE incorporation of Rb into the crystal, as well as some Sr 86 which is non-radiogenic and of course some radiogenic 87 Rb as well.

Now, if we make a plot of 87/86 Sr measured in each mineral against the current ratio of 87Rb/86Sr in each mineral , then the plot gives a straight line, such that the slope is proportional to the age of crystallization and the intercept gives the initial 87Sr/86Sr ratio. The intercept accounts for the radiogenic Sr that was made ELSE WHERE before the event that gave rise to the two minerals that ppt from the liquid.

This diagram is called the isochron diagram and you can look it up on wiki! or here