\documentclass{article}

\usepackage{pgf}
\usepackage{pgfcalendar}

\begin{document}

\pgfcalendardatetojulian{\year-\month-\day+10}{\mycount}
\pgfcalendarjuliantodate{\mycount}{\myyear}{\mymonth}{\myday}
\edef\isodate{\myyear-\mymonth-\myday}
\isodate

\end{document}