Vvccx

Eternity is namely immutability, and it can be said that eternity rejects all change. Things that possess eternity indefinitely don't change, and also cannot be interfered by things outside of it. Counting from when Kaguya came to Gensokyo, she has lived hidden away definitely for hundreds of years, but until the eternal night incident, she continued having an existence in the interior of the Bamboo Forest of the Lost without anybody knowing about them. Also, the pure Japanese-style building Eientei can be seen not to have worn out at all, which can be said to (have been) due to this ability of eternity. At least as of now, since for Eientei, the change of "others coming to visit" so to speak, has become an ordinary daily occurrence, it has already become non-eternal.[1]

Incidentally, as the Hourai Elixir made by Kaguya's ability of eternity is something that makes the consumer have a "body that rejects all change", it can't simply be said to be a kind of maximised, self-healing ability (due to this, it's said that a the whole-body can recover from a single hair, as for Fujiwara no Mokou). In the last spell card "End of Imperishable Night" which used this ability, it destroyed Reimu Hakurei and the others' spell of eternity, and forcibly called for dawn (End of Imperishable Night -Rising World-, where "Rising World" was a homophone to "dawn" as "yoake" (夜明け)).

With "instantaneous", it's namely the gaps, the instant that's impossible to recognise the existence of anything else. Kaguya, in being able to manipulate the instantaneous, is able to mish-mash the needed "instants" and make them her own time. However, whatever she gathers together, since the fact that they're the "unrecognisable instant" can't be changed, she has no means of knowing what's within the instant. For this reason, in manipulating the instantaneous, Kaguya's able to spend "time the same as others'" and "time different from others'" simultaneously. Hieda no Akyuu presented this as "having parallel timelines at the same time." In the nineteenth chapter of Silent Sinner in Blue, as the instantaneous is the smallest unit of time, time is able to accumulate countless instants. Watatsuki no Toyohime explained that to see time as continuous is due to the inability to recognise the instant.

Since her ability can be said to be the ability to manipulate "complete stop = eternity" and "extreme speed-up = instant", Akyuu also summarised it as an "ability to manipulate time". This seems very similar to Sakuya Izayoi's "ability to manipulating time".