You should get your records from CBSA. Its free and fill your application accordingly to avoid any conflicts.Out of curiousity: you need buffer because you are not sure about your travel dates from the past or because sometimes even if your dates are correct it does not match official records because the officer made a typo or something?
If it is the latter, it is quite scary as I applied with no buffer (I put all dates based on my boarding passes and tickets which I all kept for this day).