Your row is defined as a \parbox. The \parbox always starts in vertical mode. This can be shown by the following example.
\documentclass{article}
\begin{document}
\parbox{8cm}{\ifvmode vmode \else hmode \fi}
\end{document}
That means a simple next box will continue the previous vertical list without switching modes. You need to start the paragraph explicitly in horizontal mode to use a box e.g. \hphantom. This is explained in the previous answer Function and usage of \leavevmode. More information are given in the TeX Book:
Simply saying \hbox{...} won’t work, since that box will just continue
the previous vertical list without switching modes. You need to start
the paragraph explicitly, and the straightforward way to do that is
to say \indent\hbox{...}. But suppose you want to define a macro that
expands to an hbox, where this macro is to be used in the midst of a
paragraph as well as at the beginning; then you don’t want to force
users to type \indent before calling your macro at the beginning of a
paragraph, nor do you want to say \indent in the macro itself (since
that might insert unwanted indentations). One solution to this more
general problem is to say ‘\␣\unskip\hbox{...}’, since \␣ makes the
mode horizontal while \unskip removes the unwanted space. Plain TEX
provides a \leavevmode macro, which solves this problem in what is
probably the most efficient way: \leavevmode is an abbreviation for
‘\unhbox\voidbox’, where \voidbox is a permanently empty box register.
\documentclass{article}
\usepackage{tabularx}
\begin{document}
\parbox{8cm}{\ifvmode vmode \else hmode \fi}
\noindent\begin{tabularx}{\linewidth}{p{2.8cm} X}
1/00\,--\,2/00 & text text \\
\leavevmode\hphantom{0}1/00\,--\,2/00 & text text \\
01/00\,--\,2/00 & text text \\
10/00\,--\,11/00 & text text
\end{tabularx}
\end{document}
As mentioned by Werner: Use \noindent before you start the tabularx. The 15pt is the regular paragraph indent that tabularx is pushed to the right. This avoids the 15pt overfull \hbox.
\leavevmode\hphantom{0}– Marco Daniel Apr 02 '13 at 17:57\noindentbefore you start thetabularx. The 15pt is the regular paragraph indent thattabularxis pushed to the right. Also, you could use\mbox{}\hphantom{0}. – Werner Apr 02 '13 at 18:16