Proposed by Goran Topic: http://balance.wiw.org/~jkominek/lojban/9501/msg00017.html
It's a way of expressing "ago", as in giving the specific amount of time it has been since something happened. (It would work equally well for the future.) This is useful, and it has been noted before that Lojban does not have an explicit way of doing this, but I think that what we have now - phrases like pu pa jeftu - work well enough. Literally, "before something which is a week long", where the thing is assumed from context to be the time interval between when it happened and now.
Similarly, "30 kilometers north of here" would be be'a pa ki'otre be li cino, where the thing which is 30 kilometers long is assumed to be the distance from here to there.
Unfortunately, it's not entirely clear; pu pa jeftu could refer to "before a week-long vacation", and be'a pa ki'otre be li cino could mean just on the north side of a road which is 30 km long. Perhaps this could be avoided by using pu le jeftu and be'a le ki'otre be li cino instead.
The alternative is termsets, which are evil and wordy, and for which the Book gives too few examples.
A better alternative is to use the space and time distance tags (VA and ZI) to tag space and time distances. So za lo jetfu for "a week ago" or "a week hence" (the tense determines which) and va lo ki'otre be li cino for "30 km away" (in a direction determined eventually by a FAhA). -xorxes
I suppose that it would be clear from context whether you are using the tags like that. However, as the list message points out, vi le broda is often used to mean "near the broda" - though zi isn't usually used that way (on the other hand, ca is used for time, but rarely bu'u for space.) This is partially the cmavo list's fault, giving "vi ma" and "ca ma" for "where?" and "when?". So the same problem results - va lo ki'otre be li cino could mean "in the general area of the road that is 30 km long".