You can experiment with the first and fifth (optional) argument to obtain the result that you wish. Please compare:
\documentclass{article}
\usepackage{chronology}
\begin{document}
\begin{chronology}[1]{2014}{2018}{\textwidth}
\event{\decimaldate{1}{7}{2014}}{Hello}
\event{\decimaldate{20}{1}{2018}}{NOW}
\event{2017}{test}
\end{chronology}
\begin{chronology}[5]{2014}{2018}{\textwidth}[0.3\textwidth]
\event{\decimaldate{1}{7}{2014}}{Hello}
\event{\decimaldate{20}{1}{2018}}{NOW}
\event{2017}{test}
\end{chronology}
\DeclareDocumentCommand \event {o m m}{%
\pgfmathsetlength\xstop{(#2-\theyearstart)*\unit}%
\IfNoValueTF {#1} {%
\ifflipped%
\draw[chronevent]%
(\xstop, 0) circle (.1\unit)% % was 0.2 -- PS
node[flippedeventlabel] {#3};%
\else%
\draw[chronevent]%
(\xstop, 0) circle (.1\unit)% % was 0.2 -- PS
node[eventlabel] {#3};%
\fi%
}{%
\pgfmathsetlength\xstart{(#1-\theyearstart)*\unit}%
\ifflipped%
\draw[chronevent,rounded corners=.2\unit]%
(\xstart,-.2\unit) rectangle%
node[flippedeventlabel] {#3} (\xstop,.2\unit);%
\else%
\draw[chronevent,rounded corners=.2\unit]%
(\xstart,-.2\unit) rectangle%
node[eventlabel] {#3} (\xstop,.2\unit);%
\fi%
}%
}
\begin{chronology}[1]{2014}{2018}{\textwidth}
\event{\decimaldate{1}{7}{2014}}{Hello}
\event{\decimaldate{20}{1}{2018}}{NOW}
\event{2017}{test}
\end{chronology}
\end{document}

Changes in the lines marked PS are only exemplary. You can set ticks and lines with rounded ends in a similar manner.