Create a regular sequence of date-times (POSIXct) using seq()

Question

My goal is to create a vector of POSIXct time stamps given a start, an end and a delta (15min, 1hour, 1day). I hoped I could use seq for this, but I have a problem converting between the numeric and POSIXct representation:

now <- Sys.time()
now
# [1] "2012-01-19 10:30:39 CET"
as.POSIXct(as.double(now), origin="1970-01-01", tz="CET")
# [1] "2012-01-19 09:30:39 CET"
as.POSIXct(as.double(now), origin=as.POSIXct("1970-01-01", tz="CET"), tz="CET")
# [1] "2012-01-19 09:30:39 CET"

One hour gets lost during this conversion. What am I doing wrong?

Answer 1

There is a seq() method for objects of class "POSIXt" which is the super class of the "POSIXlt" and "POSIXct" classes. As such you don't need to do any conversion.

> now <- Sys.time()
> tseq <- seq(from = now, length.out = 100, by = "mins")
> length(tseq)
[1] 100
> head(tseq)
[1] "2012-01-19 10:52:38 GMT" "2012-01-19 10:53:38 GMT"
[3] "2012-01-19 10:54:38 GMT" "2012-01-19 10:55:38 GMT"
[5] "2012-01-19 10:56:38 GMT" "2012-01-19 10:57:38 GMT"

Answer 2

You have to be aware that when converting from POSIXct to numeric, R takes the timezone into account but always starts counting from a GMT origin :

> xgmt <- as.POSIXct('2011-01-01 14:00:00',tz='GMT')
> xest <- as.POSIXct('2011-01-01 14:00:00',tz='EST')
> (as.numeric(xgmt) - as.numeric(xest)) / 3600
[1] -5

As you see, the time in EST is conceived to be five hours earlier than the time in GMT, which is the time difference between both timezones. It's that value that is saved internally.

The as.POSIXCT() function just adds an attribute containing the timezone. It doesn't alter the value, so you get the time presented in GMT time, but with an attribute telling it is EST. This also means that once you go from POSIXct to numeric, you should treat your data as if it's GMT time. (It's a whole lot more complex than that, but it's the general idea). So you have to calculate the offset as follows:

> nest <- as.numeric(xest)
> origin <- as.POSIXct('1970-01-01 00:00:00',tz='EST')
> offset <- as.numeric(origin)
> as.POSIXct(nest-offset,origin=origin)
[1] "2011-01-01 14:00:00 EST"

This works whatever the timezone is in your locale (in my case, that's actually CET). Also note that behaviour of timezone data can differ between systems.