using ddply seems to wipe out the "Duration"

plyr_lubridate_test.R

library(plyr)
library(lubridate)

n_intvl <- 48
dur_intvl <- dseconds(900)

intvl_15min <- data.frame(
  start = ymd("2001-01-01", tz="America/Chicago") + seq(0, n_intvl-1)*dur_intvl,
  dur = rep(dur_intvl, n_intvl)
)

str(intvl_15min)
# note that dur has class Duration

intvl_hour <- ddply(
  .data = mutate(intvl_15min, start=floor_date(start, "hour")),
  .variables = .(start),
  .fun = summarize,
  dur = sum(dur)
)

str(intvl_hour)
# note that dur has class numeric