Is there a way to assign DST transitions automatically in lubridate?

Question

I have date-time pairs in a csv file that look like

11/4/2012

in one column and

12:06:08 AM

in the neighboring column. They are recorded in local time (i.e., they switch to PST and PDT at the appropriate times), but there is no tz or DST indicator in the file. The only visible way to detect that is that the sequence of times does funny things. For example, on November 4, 2012, I have a sequence of times like

12:51:20 AM 1:13:08 AM 1:24:58 AM 1:40:28 AM 1:48:08 AM 1:54:08 AM 1:56:58 AM 1:04:28 AM 1:05:48 AM 1:07:18 AM 1:15:00 AM 1:39:08 AM 2:05:38 AM

PST presumably begins with the 1:04:28 AM reading, but there is no indicator.

Is there a straightforward approach to assigning time zones properly (presumably using lubridate)? The file is long, so I'd rather not loop through one reading at a time, as I fear that could take some time. I'll have to do the same thing in reverse for the spring.

Answer 1

This isn't possible. There's no way to know with certainty that "11/4/2012 1:04:28 AM" is PST and not actually an observation between "11/4/2012 12:51:20 AM" and "11/4/2012 1:13:08 AM" PDT.

If you're certain the observations are ordered in the file, you could convert them to POSIXt and take the diff of the vector. Any negative values will be DST changes. You may miss some, however, if the time between observations across a DST change is greater than 1 hour.

Lines <- "11/4/2012 12:51:20 AM
11/4/2012 01:13:08 AM
11/4/2012 01:24:58 AM
11/4/2012 01:40:28 AM
11/4/2012 01:48:08 AM
11/4/2012 01:54:08 AM
11/4/2012 01:56:58 AM
11/4/2012 01:04:28 AM
11/4/2012 01:05:48 AM
11/4/2012 01:07:18 AM
11/4/2012 01:15:00 AM
11/4/2012 01:39:08 AM
11/4/2012 02:05:38 AM"

x <- scan(con <- textConnection(Lines), what="", sep="\n")
close(con)
diff(strptime(x, format="%m/%d/%Y %I:%M:%S %p"))
# Time differences in mins
#  [1]  21.800000  11.833333  15.500000   7.666667   6.000000   2.833333
#  [7] -52.500000   1.333333   1.500000   7.700000  24.133333  86.500000