In the "Data Types Information Model" specification, in "7 Date Time Package", section "7.1.2 Design", under the "General Approach" heading it states, "Date and Date_time measure calendrical time from the date 0001-01-01 ..."
This makes sense. It's inconsistent, however, with "7.2.2 DV_DATE Class":
ensure Result >= 0
Numeric value of the date as days since the calendar origin point 1/1/0000
This doesn't make sense. There is no such year in historical time as the year zero, and it's inconsistent with the earlier statement that the origin is in the year 1.