That's as succint as I can get it!

This is peculiar to Spain, I think. The accreditation conferred by the "visado" certainly doesn't endorse the plans - planning permission is given in a "Licencia de Obras" issued by local government. In this context, the "visado" is issued by the Colegio de Arquitectos for that province and is an endorsement of the professional capacity of the architect or quantity surveyor charged with the inspection and overseeing of the works:

"El visado colegial constituye una de las funciones atribuidas a los Colegios Profesionales por el art. 5 de su Ley reguladora. [...] Supone «un acto corporativo de naturaleza interna» (STS 27 diciembre 1989), o, dicho de otro modo, «un acto colegial de control de la actividad profesional de los colegiados». [...]

"El objeto del visado es comprobar, al menos:

a) La identidad y habilitación profesional del autor del trabajo, utilizando para ello los registros de colegiados previstos en el artículo 10.2.

b) La corrección e integridad formal de la documentación del trabajo profesional de acuerdo con la normativa aplicable al trabajo del que se trate."
http://www.arquitectura-tecnica.org/EJERVISADO.htm

So here I think the sentence you quote is referring to the time elapsed between obtaining the "visado de dirección de obra" - which gives the promotor of the works the go-ahead to apply to the town hall for the building licence - and registering the completed works in the Property Register. Of course, in practice, works may be completed long before they are registered, in rural Spain, at least (where my experience lies).

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Note added at 7 hrs (2012-10-30 23:29:31 GMT)
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What I don't fully understand (and I haven't read the full article!) is why the author has chosen to use the date of the "visado de dirección de obra" instead of the date of issue of the building licence. Perhaps the data on the "visados" was easier to obtain ...