Hi,
You are correct that the start and end dates should auto-complete based on the free text date field. However, this feature depends on javascript, so if you have javascript turned off, the feature will not work and you would have to enter the dates manually.
It would be interesting to know if you have changed anything recently or has this been a long running problem? Is this a new AtoM install? Was it misbehaving before? What browser are you using and have you tried another?
Let us know more!
best regards,
Corinne
Artefactual Systems