require(zoo)
require(chron)

fmt = "%m/%d/%Y %H:%M"
tail1 = function(x) tail(x, 1)

old = read.zoo("old.txt", header = F,
   FUN = as.chron, format = fmt,
   colClasses = c(NA, "numeric", "NULL"),
   sep = "\t", aggregate = tail1)

new = read.zoo("new.txt", header = T,
   FUN = as.chron, format = fmt,
   sep = ",",
   colClasses = c("NULL", NA, "numeric",
      "NULL", "NULL",
      "NULL", "NULL", "NULL", "NULL", "NULL", "NULL", "NULL",
      "NULL", "NULL", "NULL", "NULL", "NULL", "NULL"),
   aggregate = tail1)

min = seq(start(new), end(new), by = times("00:01:00"))
intData = na.approx(new, xout = min)

min = times("00:15:00")
offset = min*1/2

avgData = suppressWarnings(aggregate(intData,
   trunc(time(intData) + offset, min), mean))