date              Rank         new_Weight
2019-01-01         20           2
2019-01-01         30           5
2019-01-01         10           8
2019-02-02          3           10
2019-02-02          5            2
....               ...          ....

by(df, df$date,subset) fuunction(x){
  x<-df$rank*df$new_weight/sum(df$new_weigth)
}