This research examines the effect of time of testing on adult age differences in resistance to interference, working memory, processing speed, and vocabulary. Results show that time of testing modulates age-related differences only in the ability to resist automatic and prepotent responses. Older adults tested in the afternoon were more susceptible to interference than young adults tested at the same time of the day, and than their peers tested in the morning. In contrast, age-related differences in working memory, processing speed, and vocabulary were not modulated by time of the day. Our findings suggest that age-related modulation of performance as a function of the time of the day is specific to resistance to interference.