Hi Esuna
"the ability of a civilization to "stop" time for the general population"
Far, far in the future they solve the issue of 'time'
They can move freely into the past or the future
At that point, time no longer affects them. They are outside of time in that they can control it
In your terms, they 'stop time for the general population'
There is no issue of operating machinery or making changes to the environment
These are all operated, in the far, far future, by mental effort. Force of will. The mind
They have built doorways which allow them to go back into the past.
In the far, far future everything is near-enough perfect, so the only way they can develop as beings is to go back into the past and experience the hardships of life as it was
They can look into the past by means of video screens that enable them to rove and see events and lives happening
They use these to select the person they want to be
Once that is done, they arrange to be born as that person, and experience his or her life
Today, in 2015, many people know that this sort of thing happens all the time. For want of a better name, they call it 'reincarnation'
Let me know if you need any further details
Best wishes