Match a time datum with the nearest time in a vector of times

closestTime.R

### Function to match a time datum with the closest time in another (for, say, finding water heights)
closest.time <- function(in.datum, ref.vec){
  return(which.min(abs(difftime(in.datum, ref.vec, units="secs"))))
}

There must be a better way to do this. I need to take an "algorithms" class.