The only problem is that we only know the number of daughter atoms now present, and some of those may have been present prior to the start of our clock. The reason for this is that Rb has become distributed unequally through the Earth over time.
The age of a rock can be calculated if its ratio of uranium to lead is known.
This is unstable and eventually decays to lead, which is stable.
Uranium-Lead dating is a radiometric dating method that uses the decay chain of uranium and lead to find the age of a rock.
As uranium decays radioactively, it becomes different chemical elements until it stops at lead.
Of all the isotopic dating methods in use today, the uranium-lead method is the oldest and, when done carefully, the most reliable.