My understanding is that proper time/length/mass is the time/length/mass measured by an object in its own F.O.R. (the pronumerals with the nought subscript in the three equations), so I'm a tad puzzled as well, but maybe the it's trying to say that there's not a "proper" time in the sense that there's no one "correct" frame of reference to measure things from.
I may be wrong, I missed out on a few lessons at the end of the year and learning at home just isn't the same.
EDIT: Care to elaborate Kimyia?