Using your example, I put 17-Mar-16 in B2, 01-Mar-16 in A3, formula for Days to/from Due Date in B3 and Date done in C3.
Formula would then be:
So if Date done is not entered (which is what "" means), then the report date (17-Mar-16 in B1) will be subtracted from Due date (01-Mar-16 in A3).
But if Date done has been entered then, the Date done will be subtracted from the Due date.
For Report date you will use =TODAY(), which will change every day and thus Days to/from Due Date will change. For Date done you will manually input date (fixed value), thus the Days to/from Due Date will remain the same once Date done is entered.