\subsection{Until Element}
\label{sec:until}

The \xmlelement{until} element indicates how long the current basic
status (open or closed) is likely to be valid, expressed as an absolute
time.

This indication allows the watcher to make better decisions.  For
example, if a presentity indicates that it is likely to be unreachable
for an extended period of time, the watcher may decide to request
assistance from somebody else, rather than waiting for the presentity to
return. 

Often, the duration of the status information is not known precisely. 
Thus, it is helpful to indicate the precision, here expressed in
seconds.  For example, an absence of ``a few hours'' can easily be
expressed as a time some hours into the future, with a precision of 7200
seconds.

\motivation{An absolute time was chosen to simplify integration with
calendaring applications.  This combination appears to be sematically
cleaner than enumerating various measurement units such as ``months'',
``weeks'', ``days'' or ``hours''.}

Both the \xmlelement{from} and \xmlelement{until} information might be
derived from calendar information, reflecting the start and end time of
an activity.  (Examples include the Date Time Start and Date Time End
properties of RFC 2445.  For simplicity, RPIDS only supports single
events, without repetition.)

Any statements such as anticipated validity are not historical facts and
are forward-looking statements that involve risks and uncertainties;
actual results may differ from the forward-looking statements. 
