Hello,
have you tried putting an ISO 8601 formatted datetime string as the <default>-value in the model? This should work since Alfresco will try to convert the text of the <default>-value into the type of the property and for d:date / d:datetime it should use ISO 8601 date format parsing. So try putting 1900-01-01T00:00:00.000Z into your <default>-value.
Regards
Axel