When a moodle site has its timezone set to anything other than default: Server's local time, there is an offset that occurs when setting times for due dates.

For example the server is in a different timezone UTC-5, and the settings in Moodle are set to timezone UTC-8 in the location settings (TimeZoneSettings.png). In order for times to show correctly you need to manually add 3 hours for the time to show correctly, (TimesSetToNoon.png) else the time is off by the difference (TimesShowAsNine.png).

See attached photos