Thanks to peterleinchen I have solved the Clock app issue.
He posted a solution in Sailfish Core Apps do not work after Update to Sailfish 4.0.1.45 (sandboxing problem?)
I have copied it here.
Using terminal, type the following commands.
devel-su
pkcon install --allow-reinstall jolla-clock
I should also add you will need to remove the dynamic icons before the reinstall process.
This is done by also using Terminal.
devel-su
pkcon remove harbour-dyncal
pkcon remove harbour-dynclock
See my first post in this thread for the removal and reinstallation of the Calendar app.