I have observations for individuals at two points in times. I want to model the probability of smoking at time t, giving characteristics at time t-a, so the model looks like this:

Smokingt = B0 + B1*Smokingt-a + BiXi

The problem is that for some individuals, the duration "a" between two observations is 2 years, for other it's 3 years and for some others 4.

Since I want a model that can predict the annual probability to stop smoking (or to start smoking for non smokers), I am thus wondering what should I do. Can I just add in the model a linear regressor for the number of years between two observations? The model would thus be:

Smokingt = B0 + B1*Smokingt-a B2*a + BiXi