General formula for time t used in carbon14 dating is 57300693lnn_tn_0 if a fossil has say 25 of carbon14 as compared to living sample than it is 11460 years old as it has onefourth carbon it is 5730211460 years old as in the above example n_tn_0025 and ln0251386 t5730069313865730211460 years.