Demographic data revolves around dates and ages
Demographic analysis is primarily concerned with capturing dates and ages at key events: birth, marriage, death, and at describing the population at specific dates in terms of its composition (e.g. never married, currently married, previously married).
In surveys information on current status and timing of events is collected directly from the respondents. Current status information (e.g. are you married?) is usually simple to collect, but finding out about the timing of events (e.g. when did you marry? how old were you when you married?) can be more complicated as it involves retrospective questions requiring respondents to remember past events.
Respondents may have poor recall of ages, dates and events. This will reduce the quality of the retrospective data. The quality of current status data may be affected by unwillingness of respondents to admit to their status (e.g. divorced people may report themselves as never-married).