I always thought this was when the installed version of the Identity Application was created at the developer side... This is the date at the 4.9 version

Dashboard last updated at: Oct 16, 2017 2:52:56 PM

So that also does not align, but intriguingly it is a bit different than yours....

The "Dashboard last updated" refers to the last time the logged in user edited and saved their dashboard.  When the user updates their dashboard it gets saved to the dashboard preferences (srvprvDashboardPrefs) attribute on their eDirectory object and you can see the timestamp stored there.  However, if the user has never updated their dashboard then the dashboard preferences attribute wont exist and the timestamp will reflect the original (install?) date/time.