declare
   @dtEnd datetime =
   case
      when cast(GetDate() as TIME) < '6:00' then DateAdd(Hour, -6, dateadd(Day, DateDiff(Day, 0, GetDate()), 0))   -- 6pm yesterday
      when cast(GetDate() as TIME) < '18:00' then DateAdd(Hour, 6, dateadd(Day, DateDiff(Day, 0, GetDate()), 0))   -- 6am today
      else DateAdd(Hour, 18, dateadd(Day, DateDiff(Day, 0, GetDate()), 0))   -- 6pm today
      end

declare
   @dtStart datetime = dateadd(Hour, -12, @dtEnd) 

select
   <whatever>
from
   MyTable
where
   MyDatetime between @dtStart and @dtEnd