Neuropsychological assessment plays a prominent role in the diagnosis of Alzheimer's disease (AD) and other cognitive impairments. Increasingly, neuropsychological test results are also used to guide clinicians in the prescription of anti-dementia medication. There is evidence to suggest that the cognitive ability of an individual with AD may vary over the course of a day. If time of day can influence an individual's performance on cognitive tests, then it could potentially affect his or her diagnosis and eligibility for treatment. This study set out to explore the effect of time of day on Mini-Mental State Examination (MMSE) performance in individuals with AD and Age-Associated Cognitive Decline (AACD). No significant effect of time of day (TOD) on Folstein MMSE performance was found. However, some interesting results were highlighted and future research suggested. Overall, the study does not provide evidence that time of day needs to be considered when interpreting the result of a short cognitive screening test.