Lookup values corresponding to the closest date

Question

I have a data.frame x with date and Value

x = structure(list(date = structure(c(1376534700, 1411930800, 1461707400, 
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Answer 1

sapply(y, function(z) x$Value[which.min(abs(x$date - z))])
# [1] 40 65 44 67 44

Answer 2

Using data.table you can join to the nearest value

library(data.table)

x <- as.data.table(x)
y <- data.table(date=y)

res <- x[y, on='date', roll='nearest']