It looks like the “last active” stat on user profiles registers the last date of log-in / simply viewing iNaturalist and not the last date of activity like adding an ID, comment, observation, or other publicly accessible content associated with a profile.
For privacy purposes, I would suggest:
–rounding it off to month, e.g. December 2018 rather than December 18, 2018, and/or
–basing the date on last publicly accessible content (IDs, comments, observations) rather than last date logged in, sending private messages, etc