For those needing portability, Rad Pro for Desktop works with Windows 8.1 tablets (just tested). Will not work with Surface tablets running Windows RT.
Days Select Element Select Isotope cpm cpm Half LifeSelect Units Calculated ActivityOriginal Activity Decay CalculationsCalculation Date and/or TimeOriginal Date and/or Time   
Radioactive Decay   
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Useful for calculating today's activity for any radioactive isotope. You may also back decay sources to find out the original activity (or for any date), knowing the current activity. If the isotope that you wish to decay is not on the drop down list, check the 'not listed' check-box and manually enter the isotope name and its half-life to perform the calculation. You may enter date or time or a combination of both. Calendar pickers are included for date entry convenience.