About your 10 o'clock thing, there are only a few select time periods you can travel too. And time passes at the same speed for 65,000,000 B.C. and 1000 A.D. and all the other time periods.
Also, perhaps there are portals Lucca's gate key can not access. Would explain Magus going to 12,000 B.C. in a portal that takes people to 65,000,000 B.C.
However something like the Time Egg could overpower this little rule of time traveling. The Time Egg goes to any specific time you desire, that little Masamune thing you said, The Time Egg if someone used it would actually make it possible.