Referring to dates other than \today

See the datenumber and advdate packages (and the datetime package for formatting options).

Edit: Added small example, essentially stolen from the datenumber documentation.

\documentclass{article}

\usepackage{datenumber}

\begin{document}

\setdatetoday
\addtocounter{datenumber}{7}
\setdatebynumber{\thedatenumber}

Today is \today, 7 days from now it's \datedate.

\end{document}

\documentclass{article}
\makeatletter
\def\dayinmonth#1{%
  \ifcase#1 31\or28\or31\or30\or31\or30
            \or31\or31\or30\or31\or30\or31\fi}
\newcommand\Today[1][0]{%
  \advance\day by #1
  \edef\DiM{\dayinmonth{\the\month}}
  \ifnum\day>\DiM 
    \day=\numexpr \the\day-\DiM\relax 
    \advance\month\@ne
  \fi  
  \today}
\makeatother

\begin{document}

\today -- \Today[30]

\end{document}

the year can also be tested if the month is 13 after increasing

It is perhaps worth showing an example using the advdate package, mentioned by Villemoes, as it has a single command solution for this, \DayAfter[n], which prints the date n days ahead and leaves \today unchanged on exit. So for 7 days ahead:

\documentclass{article}
\usepackage{advdate}
\begin{document}

\today\\
\DayAfter[7]\\
\today\\

\end{document}

gives

     August 2, 2011
  August 9, 2011
  August 2, 2011

the advdate package also provides separate macros \AdvanceDate to change the value of \today to a number of days ahead together with \SaveDate and \SetDate to save and set the current value used for \today. texdoc advdate gives full details for these and other macros in the package, as usual.