How to Remove Error in Date Field

If you are wanting to change all null values to a specific date (like 11/11/2011), then a simple replace values would work where you are replacing all null values with your ficticious date.  You can also add a custom column with the following statement:

if [Your Date Column] = null then "11/11/2011" else [Your Date Column]

If you are trying to convert everything to a date and are receiving an error message, you can also try the following statement in a custom column (if your date column is currently text because of bad data):

try Date.FromText([Your Date Column]) otherwise (however you want the errors handled)