See dojo at https://dojo.telerik.com/@GaloisGirl/akAlaDIP .
The first DateTimePicker is set as readonly and has also aria-readonly manually set to true. It is correctly read by my screen reader (NVDA).
The second DateTimePicker is only set as readonly. It is read by the screen reader editable. It is not marked as readonly in the accessibility tree (per Firefox Developer Edition 71).
I can't find a formal specification, but this page says an input with ARIA role="combobox" needs to have "aria-readonly" set in addition to "readonly".
I'm documenting this for the DateTimePicker , but the same problem occurs for ComboBox ( https://dojo.telerik.com/@GaloisGirl/egeLOxIt ) and possibly other widgets that use the ARIA role combobox.