If the todonotes package is used, either in its disabled or activated form:
\usepackage[disabled]{todonotes}
\usepackage[draft]{todonotes}
the start date (or date label) is wrongly printed to the top of the page:

I used the following code:
\documentclass[10pt,a4paper,sans]{moderncv}
\usepackage[firstyear=2010,lastyear=2018]{moderntimeline}
\moderncvstyle{casual}
\moderncvcolor{blue}
\usepackage{fontspec}
\usepackage[ngerman, english]{babel}
\usepackage[disable]{todonotes}
% personal data
\name{John}{Doe}
\begin{document}
\maketitle
\section{Education}
\tlcventry{2015}{2016}{University}{}{}{}{}
\end{document}
If the todonotes package is used, either in its disabled or activated form:
the start date (or date label) is wrongly printed to the top of the page:
I used the following code: