Background Testing of orientation to time is an important part of mental status examination. The validity of errors in different aspects of temporal orientation was examined in older hospital patients as a guide to the presence of dementia or delirium and as a measure of the severity of dementia, as defined by the Global Deterioration Scale.Methods Inpatients and outpatients attending an acute hospital underwent independent assessments by two doctors on the same day to determine orientation to time and cognitive status. Optimum cut-offs for error scores on the different aspects of temporal orientation were calculated to maximise the sum of sensitivity and specificity for detection of dementia or delirium.Results Of the 262 patients assessed, 62 (23.7%) had dementia or delirium. The best cut-offs for detection of these disorders were: any error in identifying the year, month, day of the month or day of the week; and an error of more than 1 h in identifying the time of day. Failure to identify the year correctly was the most valuable single sign of dementia or delirium (sensitivity 86% and specificity 94%); failure to identify either year or month correctly was 95% sensitive and 86.5% specific for the detection of cognitive impairment. Severity of temporal disorientation, measured using a number of approaches, was strongly associated with severity of dementia.Conclusion Disorientation to time is a useful guide to the presence and severity of dementia or delirium in older hospital patients. Failure to identify the year or month correctly is a sufficiently sensitive and specific indicator of dementia or delirium to warrant more detailed cognitive assessment in this population.