Is there a column-data-type restricted to truncated date format?

for creating a unique index with a date-column A_DATE I'm using a calculated column X_DATE as (trunc(A_DATE)):

CREATE UNIQUE INDEX UX_A_TABLE
  ON A_TABLE (
    ...,
    X_DATE
  )
/

Using A_DATE in UX_A_TABLE would fail if someone would use sysdate two times a day.
Is there a column-data-type restricted to truncated date format?

-- 
Norbert
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit
Production