Extract date from a long string - r

Question

I have a data frame where the date format is as follows:
1:9:Tue Aug 12 2014 19:25:24 GMT+0530 (IST)
I want to extract three variables day, date and time in three different columns and add it to the data frame
Day as Tue
Date as 12/08/2014
Time as 7:25:24PM
The first two numbers do not mean anything.
The dataframe consists of over 700,000 rows and I want to the new columns to replace the existing ones.

Answer 1

You should be careful about adding the datetime to your data.frame as 3 separate columns, because your 3 columns do not uniquely identify a specific datetime because you do not account for timezone. This shouldn't be a problem if all your datetimes are in the same timezone though.
s <- '1:9:Tue Aug 12 2014 19:25:24 GMT+0530 (IST)'
# If the first two numbers do not mean anything and are always separated by a
# colon, then we can remove them with the following gsub command:
s <- gsub("^[[:digit:]:]+","",s)
# Now we can convert the string to a POSIXlt object, assuming they all follow
# the format of including "GMT" before the signed timezone offset
p <- strptime(s, "%a %b %d %Y %H:%M:%S GMT%z")
The above will work even if your datetimes have different timezone offsets. For example:
# these times are the same, just in a different timezone (the second is made up)
s <- c('1:9:Tue Aug 12 2014 19:25:24 GMT+0530 (IST)',
'9:1:Tue Aug 12 2014 19:55:24 GMT+0600 (WAT)')
s <- gsub("^[[:digit:]:]+","",s)
p <- strptime(s, "%a %b %d %Y %H:%M:%S GMT%z")
# the times are the same
as.POSIXct(p, tz="UTC")
# [1] "2014-08-12 08:55:24 UTC" "2014-08-12 08:55:24 UTC"
Formatting the datetimes into the strings you want is easy; just use the format specifications in ?strptime.
data.frame(Day=format(p, "%a"), Date=format(p, "%d/%m/%Y"),
Time=format(p, "%I:%M:%S%p"), stringsAsFactors=FALSE)

Answer 2

This was a tough one. R doesn't have the best support for string and date/time functions. But I was able to get it to work with some hacks:
str <- '1:9:Tue Aug 12 2014 19:25:24 GMT+0530 (IST)';
fieldsBad <- strsplit(str,':')[[1]];
fields <- c(fieldsBad[1:2],paste0(fieldsBad[3:length(fieldsBad)],collapse=':'));
dt <- strptime(fields[3],'%a %b %d %Y %H:%M:%S');
df <- data.frame();
df[1,'Day'] <- strftime(dt,'%a');
df[1,'Date'] <- strftime(dt,'%d/%m/%Y');
df[1,'Time'] <- gsub('^0','',strftime(dt,'%I:%M:%S%p'));
df;
Shows:
Day Date Time
1 Tue 12/08/2014 7:25:24PM
Explanation of hacks:
Unfortunately, the strsplit() function does not allow specifying a maximum number of fields to produce, unlike (for example) http://perldoc.perl.org/functions/split.html in Perl, which has a LIMIT argument, which would be perfect here. So I had to sort of "over-split" and then paste the extra fields back together again on colon with paste0().
Also, the strptime() call ignores the time zone information, although fortunately still parses all it can from the input string. I tried passing the time zone information explicitly to the tz= argument, but it wouldn't recognize IST or GMT+0530 or anything I tried. But since you don't seem to require the time zone, we're ok.
Finally, no format specifier for strftime() seems to allow specifying the 12-hour time without a leading zero, so I had to use %I and call gsub() to strip it off, if present.

Answer 3

library(lubridate)
library(stringr)
d <- "1:9:Tue Aug 12 2014 19:25:24 GMT+0530 (IST)"
d <- gsub("^[[:alnum:]:]+ ", "", d)
tz <- gsub("[ +-]", "", str_extract(d, " ([[:upper:]]+)[+-]"))
strptime(d, "%b %d %Y %H:%M:%S", tz=tz)
## [1] "Aug 12 2014 19:25:24 GMT+0530 (IST)"
You'll prbly need to mapply that in a data frame context since strptime takes an atomic vector for tz. So, do something like:
dat$parsed <- mapply(as.POSIXct,
gsub("^[[:alnum:]:]+ ", "", dat$date),
format="%b %d %Y %H:%M:%S",
tz=gsub("[ +-]", "", str_extract(dat$date, " ([[:upper:]]+)[+-]")))
(that'll make dat$parsed numeric, but that's what POSIXct converts it to, so it's easy to work with)

Answer 4

I realy don't know how to do it in R, but if you get this string from js, you can do something like this:
var date = new Date('Tue Aug 12 2014 19:25:24 GMT+0530 (IST)');
console.log(date.getTime());
console.log(date.getTimezoneOffset());
get time method will return unix timestamp in ms, and getTimezoneOffset will return timezone offset in minutes. Then, you can parse it using date funcions in R. I hope, it is implemented there.