Method. A = A₀ · e^−λt, where λ = ln 2 / T½ and t is the time between the two dates (negative for back-decay). Time to reach a target: t = T½ · log₂(A₀ / A). Half-lives are from the National Nuclear Data Center, Brookhaven National Laboratory (NuDat 3, ENSDF-evaluated data); 1 year = 365.25 days. Results are for the parent radionuclide only and do not include ingrowth of radioactive progeny. Times are your device’s local time; elapsed time accounts for daylight-saving changes. 1 Ci = 3.7 × 10¹⁰ Bq; 1 Bq = 60 dpm; cpm = dpm × counting efficiency. Nothing you enter is transmitted or stored.

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