Member first Responded Date is supposed to have the date that the member was first given a "responded" member status. This works, but if you change the member status to anything (another responded value, a non responded value, or even the same value), it clears out the date. This makes the "Member First Responded Date" useless.

Follow these steps.
1. Add a contact to a campaign w/ a member status of Responded.
2. See that "Member First Responded Date" is Today's date.
3. Edit the member status of that contact, and press save.
4. See that the "Member First Responded Date" is now blank.