The battery in the MK48T02 lasts about 10 years, typically. It helps if you stopped the clock in the IC before putting it in storage
File & saw at the IC corners at pins 12 and 13 until the battery leads are exposed. Measure the residual voltage to determine the polarity of the battery leads. My results for 6 ICs were: the battery lead closest to pin 12 is always the negative. Cut out battery and superglue a 3V lithium watch battery in its place. Solder wires to the battery leads.
Pot the IC in epoxy, and you are done!