Closest date in a vector to a given date

The which.closest() function from the birk package is a simple option.

coldate= as.Date(c("2013-08-03", "2013-09-04", "2013-09-08", "2013-09-12", "2013-11-01"))
x = as.Date("2013-10-01")

which.closest(coldate, x)
[1] 4

See also the which.min function:

R> which.min(abs(x-coldate))
[1] 4

you miss an abs to take care of negative values:

which(abs(coldate-x) == min(abs(coldate - x)))
[1] 4