title: how to convert date time format "%Y-%m-%d %H:%M:%S" to "%Y-%m-%d %H:%M:%S.sss" in r dataframe? second with fraction
body: <p>I am struggling to convert date time format <code>"%Y-%m-%d %H:%M:%S"</code> to <code>"%Y-%m-%d %H:%M:%S.sss"</code>  in an R <code>data.frame</code>? Note that I want seconds with fractional seconds. </p>