na.approx function does not produce correct timestamps - r

Question

I have a large dataset of electric load data with a missing timestamp for the last Sunday of March of each year due to daylight saving time. I have copied below a few rows containing a missing timestamp.
structure(list(Date_Time = structure(c(1427569200, 1427572800,
1427576400, 1427580000, 1427583600, 1427587200, NA, 1427590800,
1427594400, 1427598000, 1427601600, 1427605200), tzone = "EET", class = c("POSIXct",
"POSIXt")), Day_ahead_Load = c("7139", "6598", "6137", "5177",
"4728", "4628", "N/A", "4426", "4326", "4374", "4546", "4885"
), Actual_Load = c(6541, 6020, 5602, 5084, 4640, 4593, NA, 4353,
NA, NA, 4333, 4556)), row.names = c(NA, -12L), class = "data.frame")
#> Date_Time Day_ahead_Load Actual_Load
#> 1 2015-03-28 21:00:00 7139 6541
#> 2 2015-03-28 22:00:00 6598 6020
#> 3 2015-03-28 23:00:00 6137 5602
#> 4 2015-03-29 00:00:00 5177 5084
#> 5 2015-03-29 01:00:00 4728 4640
#> 6 2015-03-29 02:00:00 4628 4593
#> 7 <NA> N/A NA
#> 8 2015-03-29 04:00:00 4426 4353
#> 9 2015-03-29 05:00:00 4326 NA
#> 10 2015-03-29 06:00:00 4374 NA
#> 11 2015-03-29 07:00:00 4546 4333
#> 12 2015-03-29 08:00:00 4885 4556
I have tried to fill these missing timestamps using na.approx, but the function returns "2015-03-29 02:30:00", instead of "2015-03-29 03:00:00". It does not use the correct scale.
mydata$Date_Time <- as.POSIXct(na.approx(mydata$Date_Time), origin = "1970-01-01 00:00:00", tz = "EET")
#> Date_Time Day_ahead_Load Actual_Load
#> 1 2015-03-28 21:00:00 7139 6541
#> 2 2015-03-28 22:00:00 6598 6020
#> 3 2015-03-28 23:00:00 6137 5602
#> 4 2015-03-29 00:00:00 5177 5084
#> 5 2015-03-29 01:00:00 4728 4640
#> 6 2015-03-29 02:00:00 4628 4593
#> 7 2015-03-29 02:30:00 N/A NA
#> 8 2015-03-29 04:00:00 4426 4353
#> 9 2015-03-29 05:00:00 4326 NA
#> 10 2015-03-29 06:00:00 4374 NA
#> 11 2015-03-29 07:00:00 4546 4333
#> 12 2015-03-29 08:00:00 4885 4556
I have also tried using some other functions, such as "fill", but none of them works properly.
As I am fairly new to R, I would really appreciate any suggestions for filling the missing timestamps. Thank you in advance.

Answer 1

Actually the answer is correct. There is only one hour difference between the 6th and 8th rows due to the change from standard time to daylight savings time.
Use GMT (or equivalently UTC) if you intended that there be 2 hours between those rows. Below we use the same date and time as a character string but change the timezone to GMT to avoid daylight savings time changes.
diff(mydata[c(6, 8), 1])
## Time difference of 1 hours
# use GMT
tt <- as.POSIXct(format(mydata[[1]]), tz = "GMT")
as.POSIXct(na.approx(tt), tz = "GMT", origin = "1970-01-01")
## [1] "2015-03-28 21:00:00 GMT" "2015-03-28 22:00:00 GMT"
## [3] "2015-03-28 23:00:00 GMT" "2015-03-29 00:00:00 GMT"
## [5] "2015-03-29 01:00:00 GMT" "2015-03-29 02:00:00 GMT"
## [7] "2015-03-29 03:00:00 GMT" "2015-03-29 04:00:00 GMT"
## [9] "2015-03-29 05:00:00 GMT" "2015-03-29 06:00:00 GMT"
## [11] "2015-03-29 07:00:00 GMT" "2015-03-29 08:00:00 GMT"

Answer 2

You could use the following loop which would ensure that you always get the correct answer, even if you have many NA's following each other in the data.
library(lubridate)
dat$Date_Time <- as_datetime(as.character(dat$Date_Time))
dat$id <- 1:nrow(dat)
dat$previoustime <- NA
dat$timediff <- NA
for( i in 2:nrow(dat)) {
previousdateinds <- which(!is.na(dat$Date_Time) & dat$id < i)
previousdateind <- tail(previousdateinds,1)
dat$timediff[i] <- i-previousdateind # number of rows between this row and the last non-NA time
dat$previoustime[i] <- as.character(dat$Date_Time)[previousdateind]
print(previousdateind)
}
dat$previoustime <- as_datetime(dat$previoustime)
dat$result <- ifelse(is.na(dat$Date_Time), as.character(dat$previoustime+dat$timediff*60*60),
as.character(dat$Date_Time))
dat[6:8,]
Date_Time Day_ahead_Load Actual_Load id previoustime timediff result
6 2015-03-29 02:00:00 4628 4593 6 2015-03-29 01:00:00 1 2015-03-29 02:00:00
7 <NA> N/A NA 7 2015-03-29 02:00:00 1 2015-03-29 03:00:00
8 2015-03-29 04:00:00 4426 4353 8 2015-03-29 02:00:00 2 2015-03-29 04:00:00