Your gravity update begins at the time specified in the cron. The one installed by Pi-Hole is randomly selected between 3 am and 5 am your local time, and is randomly different for all Pi-Holes to equalize server loads.
Your system log for that day (located in /var/log) will show which cron scripts ran when. On my system the following appears in the Sunday log:
May 19 03:46:01 Pi-3B CRON[5047]: (root) CMD ( PATH="$PATH:/usr/local/bin/" pihole updateGravity >/var/log/pihole_updateGravity.log || cat /var/log/pihole_updateGravity.log)
If you did a manual rebuild, or a repair, or an update then, this would be the correct time. This would not be written to the updateGravity log. The time shown is when the gravity list was changed.
Make sure you reload the page to get the updated information.