It has to do with how SSRS explicitly converts values. If the time is 00:00:00 it is assuming you want to treat the values as a date; otherwise it wants to treat the value as a datetime.

Inside the query that feeds your dataset you can convert the date value to a varchar, then replace the text '00:00:00' with '00:00:01' then convert it back to datetime. Note my example below:

DECLARE @dt datetime = '1/1/2012 00:00:00';

SELECT

date_original = @dt,

date_new = CONVERT(datetime, REPLACE(CONVERT(varchar(30), @dt, 20),'00:00:00','00:00:01'))

Results:

date_original date_new

----------------------- -----------------------

2012-01-01 00:00:00.000 2012-01-01 00:00:01.000