Dates in R

Date stored as an object is Date class

x <- as.Date("1970-01-01")
x 
# [1] "1970-01-01"
unclass(x) 
# [1] 0
unclass(as.Date("1970-01-02"))
# [1] 1

POSIXct and POSIXlt

• POSIXct: store DATE as larger integer;
• POSIXlt: store DATE as a list.

more about POSIXct and POSIXlt

• The POSIXct and POSIXlt classes allow for dates and times with control for time zones.
• Date only data, as.Date will usually be the best choice.
• POSIX classes are especially useful when timezone manipulation is important.

x <- Sys.time()
x #[1] "2016-01-14 11:52:33 CST"

p <- as.POSIXlt(x)
[1] "2016-01-15 13:52:49 CST"

names(unclass(p))
# "sec"    "min"    "hour"   "mday"   "mon"    "year"   "wday"   "yday"   "isdst"  "zone"  "gmtoff

unclass(p)
> str(unclass(p))
List of 11
 $ sec   : num 49.3
 $ min   : int 52
 $ hour  : int 13
 $ mday  : int 15
 $ mon   : int 0
 $ year  : int 116
 $ wday  : int 5
 $ yday  : int 14
 $ isdst : int 0
 $ zone  : chr "CST"
 $ gmtoff: int -21600
 - attr(*, "tzone")= chr [1:3] "" "CST" "CDT"

> t2$min
[1] 52
> weekdays(d1)
[1] "星期五"
> months(t1)
[1] "一月"
> quarters(t2)
[1] "Q1

> difftime(Sys.time(), t1, units = 'days')
Time difference of 0.01489474 days

> Sys.time()>t1
[1] TRUE

t3 <- "October 17, 1986 08:24"
t4 <- strptime(t3, "%B %d, %Y %H:%M")