P(t) = P0·e-0.00012t
P/P0 = e-0.00012t
P(t) is the amount remaining, not the amount lost. So if 34% has been lost, then 100% - 34% = 66% remains; that is P(t) = 0.66P0 and P(t)/P0 = 0.66.
0.66 = e-0.00012t
ln(0.66) = -0.00012t
ln(0.66)/(-0.00012) = t
Use your calculator to get the answer.