There are Eternals and there is Eternity, a temporal field in space-time which exists through centuries. The Eternity was created in the 24th century and does employ best minds from all of the centuries since then. It was supposed to be eternal but beyond a certain future they are blocked by some mysterious power/force. And if they travel beyond these hidden centuries, they find Earth, a barren planet & humanity completely extinct. For Eternals, who can watch all of the past(since the creation of temporal field in 24th century) present and future, by traveling upwhen & downwhen within the Eternity, the hidden centuries are a setback. They do cause reality changes and then watch its effects in the future. Their mission is to create a perfect happy time for all of humanity by minimizing their suffering through all of times. But their own time proceeds in a normal manner, they do age, and it is independent of the true reality's time changes.
In order to be really Eternal all of space-time must be closed in loop to Eternity's temporal field, but they can't. They suspect that Timers, from the hidden centuries, are fast catching up with them and want to destroy their power to change humanity's fate (a fate which they see fit). But what could the Timers be plotting against them, and why would they not let a guardian organization oversee all of humanity's welfare. Probably the answer lies in Eternity's decision to delay and sabotage space traveling capabilities of Humanity. The timers are themselves plotting reality changes to delay humanity's inevitable extinction at the specified time...but what kind of plot.
[Note: This is an old post that I had written some years ago on LiveJournal]