<when> proporciona una indicación temporal en términos relativos respecto a otros elementos determinados sobre la misma escala temporal, o en términos absolutos. 16.5.2 Placing Synchronous Events in Time
The value unknown indicates uncertainty about
the interval.
since
identifica el punto de referencia para la determinación del valor temporal del elemento <when> (cuando) en cuestión, obtenido añadiendo el intervalo al punto de referencia temporal.
Should point to another <when> element in the same
<timeline>.
Nota
If this attribute is omitted, and the absolute
attribute is not specified, then the reference point is understood to be
the origin of the enclosing <timeline>
tag.
On this
element, the global xml:id attribute must be supplied to
specify an identifier for this point in time. The value used may be chosen freely provided that
it is unique within the document and is a syntactically valid name. There is no requirement for values containing numbers to be in sequence.