Return closest date to a given date in R

It's a bit confusing since you use variables which you didn't include in your examples.

But I think this is what you want:

for (ii in 1:nrow(df))  df$birthseason[ii] <-as.character(season_enddates[which.min(abs(df$birthdate[ii] - season_enddates))])

Alternatively using lapply:

df$birthseason <- unlist(lapply(df$birthdate,function(x) as.character(season_enddates[which.min(abs(x - season_enddates))])))

Result:

> df
  ID  birthdate birthseason
1  a 2012-06-12  2012-06-21
2  b 2014-06-14  2014-06-25
3  c 2015-11-11  2015-11-16
4  d 2016-09-30  2016-11-22
5  a 2012-06-12  2012-06-21

You are looking for which season_enddate is the closest to birthdate[1], and birthdate[2], etc.

To get the data straight, I will create an actual reproducible example:

birthdate <- as.Date(c("2012-06-12", "2014-06-14", 
                       "2015-11-11", "2016-09-30", 
                       "2012-06-12"))

season_enddates <- as.Date(c("2011-11-10", "2012-11-28", 
                             "2013-11-29", "2014-11-26",
                             "2015-11-16", "2016-11-22", 
                             "2012-06-21", "2013-06-23", 
                             "2014-06-25", "2015-06-08", 
                             "2016-06-14"))

Basically I use the function you also used, except that I decided to break it down a bit, so it's easier to follow what you're trying to do:

new.vector <- rep(0, length(birthdate))
for(i in 1:length(birthdate)){
    diffs <- abs(birthdate[i] - season_enddates)
    inds  <- which.min(diffs)
    new.vector[i] <- season_enddates[inds]
}

# new.vector now contains some dates that have been converted to numbers:
as.Date(new.vector, origin = "1970-01-01")
# [1] "2012-06-21" "2014-06-25" "2015-11-16" "2016-11-22"
# [5] "2012-06-21"