I have a dataset
    id   date        y 
    8  2017-03-03    2
    7  2012-02-01    8
I want to add the number y to the date. and y is a number of year :
    id   date         
    8  2019-03-03    
    7  2020-02-01    
I tried with this date :
tmp <- as.POSIXlt(date)
tmp$year <- tmp$year + y 
date <- format(tmp)
But it's not at all the good value. 
Thank you,