Alzheimer's disease (AD) progresses from a preclinical period, through a middle phase of cognitive deterioration, to a late, profound state. The temporal progression of disability can be modeled with a horologic (time-based) function using "time-index" (TI) intervals (day- or year-units) to quantify an individual's disability across multiple cognitive and functional domains relative to a reference AD population. Clinicians and researchers can use TI quantification to assess dementia severity and initial therapy benefits. Rate of progression and confidence intervals require at least two successive measurements. Rate of progression measures can be used to support diagnosis and to investigate disease-course-modifying therapies.