The conditional operator doesn't look at what the value is being returned into. It only looks at the values it's being asked to choose between: a DateTime and null. It can't identify these as instances of the same type (because null isn't a valid DateTime), hence the error. You and I know that Nullable could do the job, but the conditional operator isn't allowed to introduce "larger" types: it's only allowed to look at the types of the two expressions it's choosing between. (Thanks to Aaronaught in comments for clarification of this point and a nice clarifying example.)
To work around this, give the operator a hint by casting the DateTime:
TextBoxActualEndDate.Text != "" ? (DateTime?)(DateTime.Parse(TextBoxActualEndDate.Text)) : null;
^^^^^^^^^^^
