It would be blank, because your I_DATEs and C_DATEs are mutually exclusive. I.e. - if one record has I_DATE, it doesn't have C_DATE and the otherwise. This happens perhaps because you are creating these two fields from one field. Hope you understand.
For any one record you would have either C_DATE or I_DATE, so not sure how you will get the difference, one of the value will be null so it might be showing the difference as null, can you chk if you are using the right logic, may be I am missing something.