If I take a source of cs137 and a geiger counter, I can measure its radioactive decay.

If I know the value of $R_0$, $t_0$, $\lambda$, and background radiation, I could solve $R(t) = R_0 e^{-\lambda t}$ for $t$, then calculate $t + t_0$.

How accurate could I get this, in terms of seconds? (drift would be zero)

How come nobody makes radioactive decay clocks?

>inb4 radioactive clocks already exist
Yes, but they don't rely on radioactive decay.