BACKGROUND: Self-report questions in substance use research and clinical screening often ask individuals to reflect on behaviors, symptoms, or events over a specified time period. However, there are different ways of phrasing conceptually similar time frames (eg, past year vs. past 12 months). METHODS: We conducted focused, abbreviated cognitive interviews with a sample of community health center patients (N = 50) to learn how they perceived and interpreted questions with alternative phrasing of similar time frames (past year vs. past 12 months; past month vs. past 30 days; past week vs. past 7 days). RESULTS: Most participants perceived the alternative time frames as identical. However, 28% suggested that the "past year" and "past 12 months" phrasings would elicit different responses by evoking distinct time periods and/or calling for different levels of recall precision. Different start and end dates for "past year" and "past 12 months" were reported by 20% of the sample. There were fewer discrepancies for shorter time frames. CONCLUSIONS: Use of "past 12 months" rather than "past year" as a time frame in self-report questions could yield more precise responses for a substantial minority of adult respondents. SCIENTIFIC SIGNIFICANCE: Subtle differences in wording of conceptually similar time frames can affect the interpretation of self-report questions and the precision of responses.