The advent of more explicit diagnostic criteria and the growing interest in "lifetime" rates of mental disorders has made imperative an accurate determination of time-related diagnostic criteria. We used data from two independent test-retest studies of the Diagnostic Interview Schedule (DIS) and the Composite International Diagnostic Interview (CIDI) to study the reliability of different time-related questions in these fully standardized diagnostic interviews. With two exceptions (anxiety disorders and alcohol-related questions), the test-retest reliability of most time-related questions in both interviews was judged to be satisfactorily high. Furthermore, the validity of time-related questions in the DIS (age at symptom onset, duration and frequency of illness episodes) was examined by comparing them with detailed "consensus" ratings done independently by different clinicians for 207 former psychiatric inpatients. A surprisingly high concordance was found for former psychotic patients except for those still severely disturbed at the follow-up investigation. Some severe restrictions were also found for nonpsychotic disorders with regard to judgment of the age at onset of phobias, panic attacks, and depression. For a more valid assessment of time-related symptom information, the use of specific memory aids is suggested.