change specific hours of xts object with POSIXct index - r

Question

I have a data frame which looks like this
df = data.frame (time = c("2013-12-23 00:00:00", "2013-12-23 00:13:00", "2013-12-23 00:14:00", "2013-12-23 00:14:01",
"2013-12-24 00:00:00", "2013-12-24 00:12:00", "2013-12-24 00:15:00", "2013-12-24 00:16:00"),
value = c(1, 2, 3, 4, 5, 6, 7, 8))
I transform this data frame to an xts object and use the POSIXct format for the index
df = as.xts(as.numeric(as.character(df[,"value"])), order.by = as.POSIXct(df[,"time"]))
What I now need is to change all the indices whose time is 00:00:00 to 22:00:00.
All other time indices must stay as they are.
The resulting object looks like this
>df
[,1]
2013-12-23 00:13:00 2
2013-12-23 00:14:00 3
2013-12-23 00:14:01 4
2013-12-23 22:00:00 1
2013-12-24 00:12:00 6
2013-12-24 00:15:00 7
2013-12-24 00:16:00 8
2013-12-24 22:00:00 5
Thanks for your help! Pat

Answer 1

We could use sub to replace the '00:00:00' to '22:00:00' in the original dataset and then do the xts conversion
df$time <- as.POSIXct(sub('00:00:00', '22:00:00', df$time),
format='%Y-%m-%d %H:%M:%S')
library(xts)
xts(df$value, order.by=df$time)
# [,1]
#2013-12-23 00:13:00 2
#2013-12-23 00:14:00 3
#2013-12-23 00:14:01 4
#2013-12-23 22:00:00 1
#2013-12-24 00:12:00 6
#2013-12-24 00:15:00 7
#2013-12-24 00:16:00 8
#2013-12-24 22:00:00 5

Answer 2

Here's a function that will shift the zero-hour of an xts object by n seconds.
shiftZeroHour <- function(x, n=1) {
stopifnot(is.xts(x))
# find zero hour
plt <- as.POSIXlt(index(x), tz=indexTZ(x))
isZeroHour <- plt$hour == 0 & plt$min == 0 & plt$sec == 0
# shift zero hour index values
.index(x)[isZeroHour] <- .index(x)[isZeroHour] + n
# ensure index is ordered properly
as.xts(x)
}
Here is how to use it with your sample data:
xdf <- structure(c(1, 2, 3, 4, 5, 6, 7, 8), .Dim = c(8L, 1L),
index = structure(c(1387778400, 1387779180, 1387779240, 1387779241,
1387864800, 1387865520, 1387865700, 1387865760), tzone = "",
tclass = c("POSIXct", "POSIXt")), class = c("xts", "zoo"),
.indexCLASS = c("POSIXct", "POSIXt"), tclass = c("POSIXct", "POSIXt"),
.indexTZ = "", tzone = "")
shiftZeroHour(xdf, 60*60*22)