Derived column error

Hi,

In my excel file that I am loading I have a column with date in this format (20150301) and another column with the time in this format too (13:08:02), at the beginning of the excel file these columns have null data and
the excel file parser assigned them as the string data type. I used Data Conversion and converted from strings to date [DT_DATE] the two columns, did I do well? Then I used derived column to convert the
null values in default values of the two columns since the database in which they will be loaded does not accept nulls, but my conversions have errors of imcompactibility of the expression conditions.
Below are the respective conversions:

Derived column Name     Deriver Column      Expression                                           Data type
dateF8       Replace 'dateF8'    ISNULL( [dateF8]  )?19000101 :[dateF8]date [DT_DATE]
dateF9       Replace 'dateF8'    ISNULL( [dateF9]  )?00:00:00 :[dateF9]date [DT_DATE]
Thanks any help