The ALGE D-line display has two banana plug inputs on the end. The Green and Black pair are for a manual button. The Yellow and Black pair are for data from a timer or pc. Sending data to the button input will create the kind of situation you describe. The newer versions of the D-line displays have little stickers on the pairs as this has been a problem area. The Yellow and Black pair is labeled "RS 232" for data.
Your D-line will act normally once the data is flowing from the COMET.
Factory set D-line displays want to display time followed by date in Euro mode. That is then overwritten by the data coming in. If the data stalls or stops being transmitted the D-line will go back to time and date. You can turn off these features with the menu selection operated by the black button on the end of the display.
Manual:
http://www.timingguys.com/pstpdf/disp_dline_led.pdf