Extract from column Date class only in R

I have a bizarre database with all sorts of date types, such as:

datfra <- data.frame(ExactDate = c(NA, NA, NA, NA, NA),
                     DateEstimated = c("1974", "22/12/1984","<1982","year?","23/05/1981" ))

> datfra
  ExactDate DateEstimated
1        NA          1974
2        NA    22/12/1984
3        NA         <1982
4        NA          year?
5        NA    23/05/1981

Would I would like to achieve is to transfer correct dates (the ones fitting dd/mm/yyyy) to the column ExactDate and leaving the unfitting ones as NA‘s such as:

> desired_datfra 
   ExactDate DateEstimated
1       NA          1974
2 22/12/1984    22/12/1984
3       NA         <1982
4       NA          year?
5 23/05/1981    23/05/1981

Any help will be welcomed!!

>Solution :

Using as.Date and format, non-matching dates automatically get discarded.

transform(datfra, ExactDate=format(as.Date(DateEstimated, '%d/%m/%Y'), '%d/%m/%Y'))
#    ExactDate DateEstimated
# 1       <NA>          1974
# 2 22/12/1984    22/12/1984
# 3       <NA>         <1982
# 4       <NA>         year?
# 5 23/05/1981    23/05/1981