High Sierra update stuck at calculating time remaining...fixed

[footman]

FWIW
My 2013 iMac tried an automatic update overnight and required a manual reboot to complete. It then got stuck with the message in the title for over an hour.
I found this fix on apple.discussions and it worked well:

1. Restart your mac in to Recovery Mode by holding down the Command (⌘) and R keys during startup.

2. Go to disk Utility and run the First Aid for Macintosh HD. Make sure your mac is connected to WIFI.

3. Restart again in the Internet Recovery Mode by holding down Command (⌘) – Option (⌥) – R. It will take some time to finish its process. When finished it will take you back to the normal recovery mode automatically.

4. Now Select Install a new copy of MAC OS. It will take 45 minutes to install here.