Thank you for all your responses. I now know that this is not associated with the custom item render.
When I bypass the item render and display the datetime in the original format the recording displays [object Object] as opposed to nothing when watching the recording.
I don't understand how this is a syncing issue, as when the room is in use all participant including ones who join late can see the datetime field for all items. When an item is created a sync message is sent, the message string is describing an object with all the fields and their values. The value for the field in question is the dateChooser.selectedDate which which should be a datetime string. Delta is set to true for all sync message so anyone entering the room late should receive all the sync messages, in the virtual room they do.
The sync message listener then creates the object as described in the sync message.
It looks as though I haven't defined what property to display properly in the recording when [object Object] appears, but it must be receiving something to display that, or else would it not be blank?