Extract year from date
As discussed in the comments, this can be achieved by converting the entry into Date format and extracting the year, for instance like this:
format(as.Date(df1$Date, format="%d/%m/%Y"),"%Y")
library(lubridate)
a=mdy(b)
year(a)
https://cran.r-project.org/web/packages/lubridate/vignettes/lubridate.html http://vita.had.co.nz/papers/lubridate.pdf