First Seen

First seen is the timestamp of a user's first recorded interaction, and it anchors the definition of a new user, the acquisition cohort and the user's age.

First seen is a deceptively simple attribute that stands behind several important decisions: a new user is someone whose first-seen falls in the selected range, and an acquisition cohort groups users by exactly this date. Two subtleties make it slippery. First, range-dependence: a user is 'new' in this week's range but 'old' in this year's, so 'new' is always relative to a window, not absolute. Second, identifier-dependence: if a visitor clears their browser storage, their identifier is reset and first-seen restarts from today; the result is that a genuinely old person is mistakenly counted as a new user and the new-user rate is overstated.