documentclass[11pt,a4paper]{article}
usepackage[utf8]{inputenc}
usepackage{amsmath}
usepackage{amsfonts}
usepackage{amssymb}
usepackage{graphicx}
usepackage[ngerman]{datenumber}
usepackage{xstring}
usepackage[left=2.00cm, right=2.50cm, top=6.50cm, bottom=3.00cm]{geometry}

newcommand{shortdat}{datedayname {,} thedateday.thedatemonth.thedateyear}
newcommand{stringtosub}{nul}
newcommand{substring}[2][1]{
    ifx\#2\
    {stringtosub}
    else
    StrMid{stringtosub}{#1}{#2}
    fi
}
    newcommand{operation}[1]{
    renewcommand{stringtosub}{#1}
    substring[1]{4}.substring[5]{6}.substring[7]{8}
    setdate{substring[1]{4}}{substring[5]{6}}{substring[7]{8}}
    shortdat
}

begin{document}
    first operation{20161025}
    \
    next operation{20161027}
end{document}