Please try:

declare @k int;

select

case when @k=0 OR NOT (@k = 0)

then cast(0 as date)

else cast('2012-04-01T10:00:00' as datetime)

end as [when] ;

and

declare @k int;

select

case when @k=0 OR NOT (@k = 0)

then cast('04/01/2012T10:00:00' as date)

else cast('2012-04-01T10:00:00' as datetime)

end as [when] ;