You still didn't answer my question. How will you measure time interval between adjacent blocks? You are describing what you are going to do with it, but my question is how will you find it?
CBlockIndex structure already have nTime field in it. I am not sure who is filling it. Even if it is not a part of transferred block header then it can be added there. Miner can set this value while he broadcast a block he mined. "Like look guys I have a block started from that block, I used this time to mine it, and there is a solution that fit under the floating threshold, feel free to use it as the base for your mining". Other clients shall only verify that the time is lower than the current time say GMT it doesn't matter what global time will be taken in account the difference in few secs is not crucial for a single block.
It can still be interesting among miners to broadcast blocks from the future and start mining from that blocks to be the first one who continue the chain if no faster block will be found. But this is already up to miners what block to choose as the base yet no block from the future shall be considered valid by clients.