The package advdate provides the command \AdvanceDate[] with an optional argument, defaulting to 1 (day). However, it changes the date output by \today. This can be prevented by using a new command, say \advanceday and using a group inside, such that registers are changed only locally, this is shown in the output, where the second call to \today (after the first call to \advanceday still shows the current date (9th of July 2014), so it is not affected by \advanceday outside!
I used an optional argument to \advanceday, defaulting to 14 days and added a command \evenmoreadvanceday, with optional argument defaulting to 28 days, but actually, \evenmoreadvanceday is redundant, since \advancedays[28] does the same job. It is rather meant to make the second date more outstanding in the code.
You can change the output format of \today by usage of datetime package and its various options/commands (not done in here)
\documentclass{article}%
\usepackage{advdate}
\newcommand{\advanceday}[1][14]{%
\begingroup
\AdvanceDate[#1]%
\today%
\endgroup
}%
\newcommand{\evenmoreadvanceday}[1][28]{%
\advanceday[#1]%
}%
\begin{document}
Today is \today~and in 14 days it is \advanceday, but today is still \today~and in 28 days it is \evenmoreadvanceday%
\end{document}

\documentclass{...}and ending with\end{document}. – Jul 09 '14 at 15:45\tomorrowcommand, but this post is about a\fortnight or a double\fortnight` command ;-). It is related, for sure, but not a duplicate. – Jul 11 '14 at 08:25