Properly align numbers with units in table

The standards of good typography do not recommend putting the values and units in the same column: this makes the table cluttered. The books I've read say that it is better to have a special column for units. Siunitx have the s column for this:

\documentclass{article}
\usepackage{siunitx,booktabs}
\pagestyle{empty}
\begin{document}
\begin{tabular}{lsS}
\toprule
Entry & \multicolumn{1}{c}{Units} & \multicolumn{1}{c}{Value}\\
\midrule
A & \day & 1\\
B & \minute & 30\\
C & \minute & 5\\
\bottomrule
\end{tabular}

\end{document}

siunitx may have features for this but...

\documentclass{article}
\usepackage{siunitx}

\def\hmm{%
  \def\day{d}\def\minute{min}%
  \def\SI##1##2{$##1\,\makebox[3em][l]{##2}$}}

\begin{document}
\begin{tabular}{>{\hmm}r}
\SI{1}{\day}\\
\SI{30}{\minute}\\
\SI{5}{\minute}
\end{tabular}

\end{document}

seems to do what you ask.