You need to have the pendant powered up before you can open sealed chests/doors. You'll know when it happens.
* These chests are in the year 600 AD and 1000 AD. Open them first in 600 AD. It will give you an option as to whether or not you want to take it now. Choose to leave it, then open it in the year 1000 AD. It will be powered up from the bad item to the good item. Then go back and get the original item in 600 AD. If you take it in 600 AD before you take it in 1000 AD, it will be gone in 1000 AD and you won't be able to get it. If you take it in 1000 AD before you open it and decide not to take it in 600 AD, you'll get the bad item instead of the good item.
Chests in 600 AD
Chests in 1000 AD
Chests in 2300 AD