R 时间戳转化

转 http://stackoverflow.com/questions/1962278/dealing-with-timestamps-in-r

You want the (standard) POSIXt type from base R that can be had in 'compact form' as a POSIXct (which is essentially a double representing fractional seconds since the epoch) or as long form in POSIXlt (which contains sub-elements). The cool thing is that arithmetic etc are defined on this -- see help(DateTimeClasses)

Quick example:

R> now <- Sys.time()
R> now
[] "2009-12-25 18:39:11 CST"
R> as.numeric(now)
[] 1.262e+09
R> now +   # adds  seconds
[] "2009-12-25 18:39:21 CST"
R> as.POSIXlt(now)
[] "2009-12-25 18:39:11 CST"
R> str(as.POSIXlt(now))
 POSIXlt[:], format: "2009-12-25 18:39:11"
R> unclass(as.POSIXlt(now))
$sec
[] 11.79

$min
[] 

$hour
[] 

$mday
[] 

$mon
[] 

$year
[] 

$wday
[] 

$yday
[] 

$isdst
[] 

attr(,"tzone")
[] "America/Chicago" "CST"             "CDT"            

As for reading them in, see help(strptime)

As for difference, easy too:

R> Jan1 <- strptime("2009-01-01 00:00:00", "%Y-%m-%d %H:%M:%S")
R> difftime(now, Jan1, unit="week")
Time difference of 51.25 weeks

Lastly, the zoo package is an extremely versatile and well-documented container for matrix with associated date/time indices.

timestamp to datestr:

datestr = as.character(as.POSIXlt(timestamp, origin="1970-01-01"))