R语言日期时间函数

Sys.Date( ) returns today's date. 显示当前日期
date() returns the current date and time. 显示当前日期和时间
# print today's date
today <-Sys.Date()
format(today, format="%B %d %Y")
"June 20 2007"

# convert date info in format 'mm/dd/yyyy'
strDates <- c("01/05/1965", "08/16/1975")
dates <- as.Date(strDates, "%m/%d/%Y") 

# convert dates to character data
strDates <- as.character(dates)


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> as.Date('1915-6-16')
[1] "1915-06-16"
> as.Date('1990/02/17')
[1] "1990-02-17"

> as.Date('1/15/2001',format='%m/%d/%Y')
[1] "2001-01-15"
> as.Date('April 26, 2001',format='%B %d, %Y')
[1] "2001-04-26"
> as.Date('22JUN01',format='%d%b%y')   # %y is system-specific; use with caution
[1] "2001-06-22"

> bdays = c(tukey=as.Date('1915-06-16'),fisher=as.Date('1890-02-17'),
+           cramer=as.Date('1893-09-25'), kendall=as.Date('1907-09-06'))
> weekdays(bdays)
      tukey      fisher      cramer     kendall
"Wednesday"    "Monday"    "Monday"    "Friday"

> dtimes = c("2002-06-09 12:45:40","2003-01-29 09:30:40",
+            "2002-09-04 16:45:40","2002-11-13 20:00:40",
+            "2002-07-07 17:30:40")
> dtparts = t(as.data.frame(strsplit(dtimes,' ')))
> row.names(dtparts) = NULL
> thetimes = chron(dates=dtparts[,1],times=dtparts[,2],
+                  format=c('y-m-d','h:m:s'))
> thetimes
[1] (02-06-09 12:45:40) (03-01-29 09:30:40) (02-09-04 16:45:40)
[4] (02-11-13 20:00:40) (02-07-07 17:30:40)

> dts = c("2005-10-21 18:47:22","2005-12-24 16:39:58",
+         "2005-10-28 07:30:05 PDT")
> as.POSIXlt(dts)
[1] "2005-10-21 18:47:22" "2005-12-24 16:39:58" 
[3] "2005-10-28 07:30:05"

> dts = c(1127056501,1104295502,1129233601,1113547501,
+         1119826801,1132519502,1125298801,1113289201)
> mydates = dts
> class(mydates) = c('POSIXt','POSIXct')
> mydates
[1] "2005-09-18 08:15:01 PDT" "2004-12-28 20:45:02 PST"
[3] "2005-10-13 13:00:01 PDT" "2005-04-14 23:45:01 PDT"
[5] "2005-06-26 16:00:01 PDT" "2005-11-20 12:45:02 PST"
[7] "2005-08-29 00:00:01 PDT" "2005-04-12 00:00:01 PDT"

> mydate = strptime('16/Oct/2005:07:51:00',format='%d/%b/%Y:%H:%M:%S')
[1] "2005-10-16 07:51:00"

> ISOdate(2005,10,21,18,47,22,tz="PDT")
[1] "2005-10-21 18:47:22 PDT"

> thedate = ISOdate(2005,10,21,18,47,22,tz="PDT")
> format(thedate,'%A, %B %d, %Y %H:%M:%S')
[1] "Friday, October 21, 2005 18:47:22"

> mydate = as.POSIXlt('2005-4-19 7:01:00')
> names(mydate)
[1] "sec"   "min"   "hour"  "mday"  "mon"   "year"  
[7] "wday"  "yday"  "isdst"
> mydate$mday
[1] 19

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Ref.
http://www.statmethods.net/input/dates.html
http://www.stat.berkeley.edu/classes/s133/dates.html