error :conversion of a char data type to a datetime data type

I have a error that is below:

(0 row(s) affected)
Msg 242, Level 16, State 3, Line 4
The conversion of a char data type to a datetime data type resulted in an out-of-range datetime value.

I have a very big sql script but this error point to a afecific area in the whole script so i am just sending you that part only. Please see below, the error is in the third line that says insert. Please help me out asap.



If EXISTS (SELECT * FROM sysobjects where name = 'ros_heent_')
      BEGIN
            insert into ros_heent_ (enterprise_id, practice_id, person_id, enc_id, created_by, create_timestamp, modified_by,modify_timestamp,
            rb_headache, visual_loss, rb_eyedischarge, rb_discharge, rb_nasaldischarge, rb_hearloss)
            select ros.enterprise_id, ros.practice_id, ros.person_id, ros.enc_id, ros.created_by, ros.create_timestamp, ros.modified_by, isnull(ros.modify_timestamp,getdate()),
            case when datediff(mm, p.date_of_birth, ros.create_timestamp)/12 >= 19 then 0 else null end as rb_headache,
            'N' as visual_loss,
            case when datediff(mm, p.date_of_birth, ros.create_timestamp)/12 < 19 then 0 else null end as rb_eyedischarge,
            case when datediff(mm, p.date_of_birth, ros.create_timestamp)/12 < 19 then 0 else null end as rb_discharge,
            case when datediff(mm, p.date_of_birth, ros.create_timestamp)/12 < 19 then 0 else null end as rb_nasaldischarge,
            0 as rb_hearloss
            from review_of_system_ ros, person p
            where ros.person_id = p.person_id and ros.nl_heent = 1
            and enc_id not in (select enc_id from ros_heent_)
      END
GO