So, let me get this straight. All market research questions that ask you to recall when you last did an activity are flawed. Memories get distorted over time. Not only could the event you are recalling happen earlier or later than in reality (telescoping bias), it might not have happened at all (false memory bias, also known rather splendidly as confabulation). If you want true, census-type behavioural information then you must use passive tools such as mobile apps or online cookies to capture real-time data analytics without having to remember when asked by an interviewer at some point in the future.