Check release date of LaTeX distribution

Question

When, automatically, documentation some code the LaTeX output gives me the,logical, warning:

Package: fixltx2e 2016/12/29 v2.1a fixes to LaTeX (obsolete)
Applying: [2015/01/01] Old fixltx2e package on input line 46.
Package fixltx2e Warning: fixltx2e is not required with releases after 2015
(fixltx2e)                All fixes are now in the LaTeX kernel.
(fixltx2e)                See the latexrelease package for details.
Already applied: [0000/00/00] Old fixltx2e package on input line 53.

So in my own case I could just leave out the line:

\usepackage{fixltx2e}

but this would give some problems for people that (unfortunately) still use an old version of LaTeX.

I found in https://tex.stackexchange.com/a/569709/44119:

Since version 2020-10-01 the LaTeX kernel provides the command \IfFormatAtLeastTF{<date>}{<true code>}{<false code>}

but this gives of course the same problems as this only exists after the 2020 release.

I also found https://tex.stackexchange.com/a/26229/44119:

\newcommand*\@iflatexlater{\@ifl@t@r\fmtversion}
\@iflatexlater{2001/09/01}{\TRUE}{\FALSE}

and applying this as:

  \newcommand*\@iflatexlater{\@ifl@t@r\fmtversion}
  \@iflatexlater{2016/01/01}{}{\usepackage{fixltx2e}}

gives me the error:

! LaTeX Error: Command \@ already defined.
               Or name \end... illegal, see p.192 of the manual.
See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...
l.15   \newcommand*@i
                      flatexlater{@ifl@t@r\fmtversion}
?
! Emergency stop.
 ...
l.15   \newcommand*@i
                      flatexlater{@ifl@t@r\fmtversion}
!  ==> Fatal error occurred, no output PDF file produced!

Normally on this site it is usual to create a MWE though I hope the information provided here is already sufficient to give a pointer to a solution.

Answer 1

Your last version doesn't work because you need \makeatletter to use @ in command names. The recommended way is to define:

\makeatletter
\providecommand\IfFormatAtLeastTF{\@ifl@t@r\fmtversion}
\makeatother

then use \IfFormatAtLeastTF throughout. \providecommand will only define the command if it doesn't exist yet, which is exactly what you want.

---

If you are writing a package or class, I wouldn't advise you to support LaTeX versions older than 3 or 4 years. TeXLive 2015 is old, and there is really no reason to use it nowadays: back then, support for older versions was important because (mainly) of Overleaf and arXiv, but both now use recent enough distributions.