You may be able to do this with rules, but you'll need a lot of them. You'd need to set rules on each of the fields that are bound to the date pickers, with conditions -- for example, if only one date picker field has a value, that should be the one in the text box. If more than one is filled out, your condition will need to compare the current field, with the rule on it, to the other fields that have dates, and then, if it less, populate the text box field with its value. I can't really think of a less complex way to do this without code, I'm sorry.