Calculating time remaining

My iMac was running macOS 10.13.1. Earlier today I received notification of an update. I let it update. After rebooting it has stalled: "Installation is in progress: calculating time remaining". That has been displayed for the past several hours. I think this update has corrupted my system.

Does anyone have any ideas where to go from here? I have already done a fresh installation (i.e. I erased the drive and installed fresh after the last update failed).