Hi Mike, you can format the datestring in another attribute and map this one.
If the receiving system expects yyyy-MM-dd it should not specify the field as dateTime, but as Date.
If you really want to send data to the other system that does not conform to the contract, you should, as Pim said, change the contract before reading it and change the field type. You could change to either Date or String in this case (problay String as I think Mendix will always append timezone info otherwise). This would mean you are not adhering to the contract, which is a bad idea for multiple reasons.