Transact SQL :: Check Constraint With UDF To Prevent Overlapping Dates?

I am trying to create a check constraint that prevents entry of a date range that overlaps any other date range in the table.  The below example script results in the error message: The INSERT statement conflicted with the CHECK constraint "ck_dt_overlap". what I am doing wrong?

CREATE TABLE t1 (d1 date,d2 date)
go
create function dbo.dt_overlap (@d1 date, @d2 date) RETURNS BIT
AS
BEGIN
 IF EXISTS (SELECT 1 from t1 where d1 between @d1 and @d2 or d2 between @d1 and @d2)
  RETURN 1
  RETURN 0
END
go
alter table t1 add constraint ck_dt_overlap CHECK (dbo.dt_overlap(d1,d2)=0)
go
insert into t1 values ('2015-01-01','2015-02-01')
insert into t1 values ('2015-01-15','2015-02-15')

--BOTH inserts result in the following message:  The INSERT statement conflicted with the CHECK constraint "ck_dt_overlap".

drop table t1
drop function dbo.dt_overlap