format a Date column in a Data Frame

try this package, works wonders, and was made for date/time...

library(lubridate)
Portfolio$Date2 <- mdy(Portfolio.all$Date2)

This should do it (where df is your dataframe)

df$JoiningDate <- as.Date(df$JoiningDate , format = "%m/%d/%y")

df[order(df$JoiningDate ),]