There are durations of the same family as the given duration which overlap it but are not contained in it.

Consider an abstractive set of such durations.
Such a set defines a moment which is just as much without the duration as within it.

Such a moment is a boundary moment of the duration.

Also we call upon our sense-awareness of the passage of nature to inform us that there are two such boundary moments, namely the earlier one and the later one.

We will call them the initial and the final boundaries.
There are also moments of the same family such that the shorter durations in their composition are entirely separated from the given duration.