If the datetime is null, SQL will display it as null. It doesn't automatically convert any null value to a non-null value. Something in the query or the display is, or the column is a varchar and = '' (rather than null), which when converted to datetime goes to the 0 date (iirc).
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability