Hi, see Teradata SQL Reference: Data type and Literars, p. 101:
Internal Representation of DATE Internally, Teradata Database stores each DATE value as a four-byte signed integer using the following formula: (YEAR - 1900) * 10000 + (MONTH * 100) + DAY where the YEAR, MONTH, and DAY components, defined appropriately for the Gregorian calendar,...
so your number is valid date 1st January 1 and you should not recieve blank or null value
As noted, -18989899 is the Teradata INTEGERDATE equivalent of date'0001-01-01'. When you use Windows local format to display that date (e.g. in SQL Assistant, see Tools/Options/General) then it is displayed as blank.