discuss.answer
I managed to find an answer for my question, but will put it here so if anyone finds this thread by searching, they will know what to do.
1) Log in to phpMyAdmin to access the modx database
2) Go to the table modx_system_settings
3) Find the date_timezone setting and edit it to remove the error
4) Log in to FTP
5) Delete the core/cache folder
6) Reload manager
I went around in circles for a while before I realised I had to delete the cache folder. Everything is working fine again now.
~KJ