documentclass{article}

usepackage[T1]{fontenc}
usepackage[USenglish]{babel}
usepackage[useregional]{datetime2}
selectlanguage{USenglish}

begin{document}
DTMdate{2019-1-15}
DTMdate{2019-1} % <- error here
2019 % I don't need special command here

end{document}