Hey CT fans...
 
I've finally gotten around to finishing this great game, i'm currently trying to do the sealed chest sidequest and i've been able to find (in 600 A.D.) and open (in 1000 A.D.) several chests. But I have run into a problem...there are a few chests that I can find in 600 A.D. but when I use the Epoch to go back to 1000 A.D. and go back to the chest location the chest(s) are gone...am I doing something wrong here? Or did I open these chests before...and just forgot about it?

I'm pretty sure that not all the chests contain the upgradable armour (like the Ruby Armour), so that a chest that exists in 600AD doesn't have a counterpart in the future. Not 100% certain, but it's like some of the standard chests that have better items if left unopened in the past and get better stuff (hi-ether to turbo-ether) etc. 
 
I can only think of about 5 sealed chests that upgrade, but it's been a while! Hope that helps a little.

The only ones that contain upgrades will ask you if you'd like to open them before you actually get the item. You must first select 'no' to leave them unopened, then travel to 1000AD to claim the contents. Any sealed chest that doesn't prompt you while opening it doesn't have an upgraded counterpart.

" The only ones that contain upgrades will ask you if you'd like to open them before you actually get the item. You must first select 'no' to leave them unopened, then travel to 1000AD to claim the contents. Any sealed chest that doesn't prompt you while opening it doesn't have an upgraded counterpart. "