I had an issue with Plex where the limit to airing time shown in the drop-down was an hour out, to fix this I had to upgrade to the latest version of Plex as this is a bug, and also I had to fix my system clock on the server.
See this issue for some more info: https://forums.plex.tv/t/limit-to-airing-time-always-an-hour-off/553640
This is how to set your clock to a time zone.
# show the time, was hour out.
sudo hwclock --show
# list time zones
timedatectl list-timezones
# set time zone
sudo timedatectl set-timezone Europe/London
sudo timedatectl set-ntp on
# show clock check its correct now!
sudo hwclock --show
Once this was done it started working.