While working with reading and writing to a list I came across the error with 'Edm.DateTime'.
I received this error when attempting to update a list item:
Cannot convert a primitive value to the expected type 'Edm.DateTime'.
Very simple issue and resolution. I had a data model that referred to a list containing a column of type date and time in SharePoint. When reading from this list my model had been set up with string data types for the most part and a date would come through fine under this circumstance.
When writing back to a list the data type needed to be Date instead. Changing this in the model and my update worked fine. Obvious really.