Indent tag of equation to the left

Question

I need an equation in which the equation itself is centered and the equation number (a custom tag) is located more to the left.

\documentclass[a4paper,12pt,numbers=noenddot]{scrartcl}
\usepackage{mathtools}
\usepackage{amsmath}
\begin{document}
%here the equation number is at the far right
\begin{align}
\tag{\text{tag}}
2 + 2 = 4
\end{equation}

%and here the equation number is indented, but the equation itself is not centered
\begin{align*}
2 + 2 &= 4 
&&\text{tag}

%and here a bracket remains on the right side
\begin{equation}
\tag{\text{tag}\qquad\quad}
2 + 2 = 4
\end{equation}
\end{document}

How can I fix this?

Answer 1

Something like this?

Employ \usetagform{indented} to switch to the alternative tag format, \mytag{...} to generate the alternative tag, and \usetagform{default} to switch back to the default tag format.

\documentclass[a4paper,12pt,numbers=noenddot]{scrartcl}
\usepackage{mathtools} % for '\newtagform' and '\usetagform' macros
\newtagform{indented}{}{\hspace*{15mm}} % set the desired indentation amount
\newcommand\mytag[1]{\tag{\llap{(#1)}}}
\begin{document}

\begin{equation} 2 + 2 = 4 \end{equation}

\usetagform{indented}
\stepcounter{equation}
\begin{equation} 2 + 2 = 4 \mytag{\theequation}\end{equation}

\begin{equation} 2 + 2 = 4 \mytag{Message of the Day}\end{equation}

\usetagform{default} % switch back to default
\begin{equation} 2 + 2 = 4 \end{equation}

\end{document}

Answer 2

If you really need this you can add the space to the command that sets the tag:

\documentclass{article}

\usepackage{amsmath}
\begin{document}

zzz
\begin{equation}
  1=0
\end{equation}


\makeatletter
\def\tagform@#1{\maketag@@@{(\ignorespaces#1\unskip\@@italiccorr)\hspace{1cm}}}
\makeatother

zzz
\begin{equation}
  1=0
\end{equation}

\end{document}

---

Or perhaps you are just looking for a quote that indents both sides:

\documentclass{article}

\usepackage{amsmath}
\begin{document}

zzz
\begin{equation}
  1=0
\end{equation}


\begin{quote}
zzz
\begin{equation}
  1=0
\end{equation}
\end{quote}


\end{document}