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R Datetime Classes

Date Objects

• The Date class is used to represent a calendar date only, not a clock time.
   
• The Sys.date function returns the current date.
   
• Use these date format specifiers for dates:
  

  %b  %B  %d  %m  %y  %Y
  

  Try them out with code like the following:
  

  > d = Sys.Date( )
  > cat(format(d, format="%B %d, %Y\n"))
  May 18, 2015
  

• Use help(strptime) to get a complete list of datetime format specifiers.
   
• Some example of the usage of Date functions:
   
  
  1. Create Date objects:
     

     > d <- as.Date("1776-7-4")
     > e <- as.Date("1856/12/25")
     > f <- as.Date("1970/1/1")
     > print(d)
     [1] "1776-07-04"
     > print(e)
     [1] "1856-12-25"
     > print(f)
     [1] "1970-01-01"
     > 

     To create dates from strings in other formats, a format specifier is required:
     

     > g <- as.Date("12.25.1956", format="%m.%d.%Y")
     > print(g)
     [1] "1956-12-25"
     

  2. Convert date to numeric. Jan. 1, 1970 is the zero date for R.
     

     > n = as.numeric(f)
     > print(n)
     [1] 1
     

     
     
  3. Get the Julian Date, which for R is the number of days since January 1, 1970.
     

     > d = as.Date(Sys.Date( ))
     > as.integer(d)
     [1] 15844
     > d = as.Date("1970-1-1")
     > as.integer(d)
     [1] 0
     

  4. Create a sequence of dates:
     

     > s <- as.Date("2009-1-1")
     > e <- as.Date("2010-12-1")
     > series1 <- seq(s, e, by=1)
     > series2 <- seq(s, e, by="day")
     > series3 <- seq(s, e, by="month")
     > series4 <- seq(s, e, by="3 months")
     > series5 <- seq(s, e, by="year")
     > print(series1)
     > print(series2)
     > print(series3)
     > print(series4)
     > print(series5)
      [1] "2009-01-01" "2009-01-02" "2009-01-03" "2009-01-04" "2009-01-05"
     [6] "2009-01-06" "2009-01-07" "2009-01-08" "2009-01-09" "2009-01-10"
     ...
     ... Lines omitted from display.
     ...
     [691] "2010-11-22" "2010-11-23" "2010-11-24" "2010-11-25" "2010-11-26"
     [696] "2010-11-27" "2010-11-28" "2010-11-29" "2010-11-30" "2010-12-01"
     > print(series2)
     [1] "2009-01-01" "2009-01-02" "2009-01-03" "2009-01-04" "2009-01-05"
     [6] "2009-01-06" "2009-01-07" "2009-01-08" "2009-01-09" "2009-01-10"
     ...
     ... Lines omitted from display.
     ...
     [691] "2010-11-22" "2010-11-23" "2010-11-24" "2010-11-25" "2010-11-26"
     [696] "2010-11-27" "2010-11-28" "2010-11-29" "2010-11-30" "2010-12-01"
     > print(series3)
     [1] "2009-01-01" "2009-02-01" "2009-03-01" "2009-04-01" "2009-05-01"
     [6] "2009-06-01" "2009-07-01" "2009-08-01" "2009-09-01" "2009-10-01"
     [11] "2009-11-01" "2009-12-01" "2010-01-01" "2010-02-01" "2010-03-01"
     [16] "2010-04-01" "2010-05-01" "2010-06-01" "2010-07-01" "2010-08-01"
     [21] "2010-09-01" "2010-10-01" "2010-11-01" "2010-12-01"
     > print(series4)
     [1] "2009-01-01" "2009-04-01" "2009-07-01" "2009-10-01" "2010-01-01"
     [6] "2010-04-01" "2010-07-01" "2010-10-01"
     > print(series5)
     [1] "2009-01-01" "2010-01-01"
     

  5. 1900 is not a leap year but 2000 is in Gregorian Calendar.
     

     > as.Date("2012-2-29")
     [1] "2012-02-29"
     > as.Date("2000-2-29")
     [1] "2000-02-29"
     > as.Date("1900-2-29")
     Error in charToDate(x) : 
     character string is not in a standard unambiguous format
     

POSIXct and POSIXlt Objects

• POSIXct and POSIXlt objects can represent both time and date information.
   
  
• Here are some more datetime packages that can be installed for R:
   
  
  chron   lubridate   mondate   timeDate
  
  
• Some examples:
   
  
  1. Create POSIXct timedate objects:
     

     > ISOdate(1978, 3, 2)
     [1] "1978-03-02 12:00:00 GMT"
     > ISOdatetime(1978, 3, 2, 14, 23, 34)
     [1] "1978-03-02 14:23:34 CST"
     

  2. Convert Date objects to a POSIXct objects:
     

     > as.POSIXct(as.Date("2012-5-20"))
     [1] "2012-05-19 19:00:00 CDT"
     > as.POSIXct(Sys.Date())
     [1] "2013-05-18 19:00:00 CDT"
     

  3. Convert POSIXct object to a POSIXlt object, which is a list containing the parts of the date
     

     > d <- as.Date("2013-5-20")
     > p <- as.POSIXlt(d)
     > p$year
     [1] 113
     > p$mon
     [1] 4
     > p$mday
     [1] 20
     > p$wday
     [1] 1
     > p$yday
     [1] 139
     > p
     [1] "2013-05-20 UTC"
     > attributes(p)
     $names
     [1] "sec" "min" "hour" "mday" "mon" "year" "wday" "yday" "isdst"
     
     $class
     [1] "POSIXlt" "POSIXt" 
     
     $tzone
     [1] "UTC"
     

  4. Get all leap seconds:
     

     > format(.leap.seconds)
      [1] "1972-06-30 19:00:00" "1972-12-31 18:00:00" "1973-12-31 18:00:00"
      [4] "1974-12-31 18:00:00" "1975-12-31 18:00:00" "1976-12-31 18:00:00"
      [7] "1977-12-31 18:00:00" "1978-12-31 18:00:00" "1979-12-31 18:00:00"
     [10] "1981-06-30 19:00:00" "1982-06-30 19:00:00" "1983-06-30 19:00:00"
     [13] "1985-06-30 19:00:00" "1987-12-31 18:00:00" "1989-12-31 18:00:00"
     [16] "1990-12-31 18:00:00" "1992-06-30 19:00:00" "1993-06-30 19:00:00"
     [19] "1994-06-30 19:00:00" "1995-12-31 18:00:00" "1997-06-30 19:00:00"
     [22] "1998-12-31 18:00:00" "2005-12-31 18:00:00" "2008-12-31 18:00:00"
     [25] "2012-06-30 19:00:00"