I have:
    \startluacode
      function modification_date(file, format)
        local f    = file
        local attr = lfs.attributes(f)

        assert (type(attr) == "table")
        if attr.mode ~= "directory" then
          if format == "long" then
            return os.date("%Y-%m-%d %H:%M",attr.modification)
          else
            return os.date("%Y-%m-%d",attr.modification)
          end
        end
      end
    \stopluacode

    \def\getdate
     {\dosingleempty\dogetdate}

    \def\dogetdate[#1]%
     {\ctalua{context(modification_date("\jobname.tex", "#1"))}}

    \starttext

    \getdate

    \stoptext

This gives:
\dogetdate [#1]->\ctalua
                         {context(modification_date("\jobname .tex", "#1"))}
l.29 \getdate[]
              
?

I tried with \getdate, \getdate{} and \getdate[].