Underpants cannot by definition be eternal - they can be sempiternal (without beginning or end / everlasting) but they can never be 'outside' of time as that would be a category error.
but The Underpants Of Eternity enable the wearer to exist "outside" of time.
but The Underpants Of Eternity enable the wearer to exist "outside" of time.
hence the name..
they never soil.
Time Travellers Don't Shit.