dynamic sql query syntax

DECLARE @p_taskentrydtfilter nvarchar(50),@taskentrydttag nvarchar(100)
declare @p_taskentrydt DATETIME

set @p_taskentrydtfilter= '>='
set @p_taskentrydt = '20050609'

select @taskentrydttag=case when (@p_taskentrydt='' OR @p_taskentrydt is null) then '' else
' and task.entrydt '+
@p_taskentrydtfilter+''+@p_taskentrydt+''' and task.entrydt <'''+dateadd(d,1,@p_taskentrydt)+'' end

as u can see am storing a CASE stmt in a variable but it gives me error saying
"Syntax error converting datetime from character string."
any ideas .Thanks