It's coming from 3rd party Joomla system plugin called HD Date (had_date). Looks like it has a bug in its usage of the onAfterRender event where it's trying to access undefined variables. Suggest unpublishing that Joomla plugin and contacting the developer.

I highly encourage everyone to carefully read any errors they receive. The vast majority reported here aren't from CB. CBSubs is aggressive with its logging behavior so we can catch and debug payment issues. It will catch 3rd party errors too if they happen whole CBSubs is processing something. Some things to look for is the pathing to the file that the error is in. In this case notice /plugins/system/hd_date. This tells you it's a Joomla plugin of type System with a name of "hd_date". Using this you can usually track down what's causing your problem.