R provides several options for dealing with date and date/time data. The builtin
as.Date function handles dates (without times);the contributed library
chron handles dates andtimes, but does not control for time zones; and the
POSIXct and
POSIXlt classes allow for dates and times with control for time zones.The general rule for date/time data in R is to use the simplest technique possible.Thus, for date only data,
as.Date will usually be the best choice. If youneed to handle dates and times, without timezone information, the
chronlibrary is a good choice; the POSIX classes are especially useful when timezonemanipulation is important. Also, don't overlook the various "
as."functions (see Section) for converting among the different date types when necessary.Except for the
POSIXlt class, dates are stored internally as the numberof days or seconds from some reference date. Thus dates in R will generally have a numeric mode, and the
class function can be used to find the way they are actually being stored. The
POSIXlt class stores date/timevalues as a list of components (
hour,
min,
sec,
mon, etc.) making it easy to extract these parts. To get the current date, the Sys.Date function will return a
Date object which can be converted to a different classif necessary.The following subsections will describe the different types of date values in moredetail.The
as.Date function allows a variety of input formats through the
format= argument. The default formatis a four digit year, followed by a month, then a day, separated by either dashes or slashes. The following example shows some examples of dates which
as.Date will accept by default:
>as.Date('1915-6-16')[1]"1915-06-16">as.Date('1990/02/17')[1]"1990-02-17" | Code | Value |
| %d | Day of the month (decimal number) |
| %m | Month (decimal number) |
| %b | Month (abbreviated) |
| %B | Month (full name) |
| %y | Year (2 digit) |
| %Y | Year (4 digit) |
If your input dates are not in the standard format, a format string canbe composed using the elements shown in Table. The following examples show some ways that this can be used:
>as.Date('1/15/2001',format='%m/%d/%Y')[1]"2001-01-15">as.Date('April26,2001',format='%B%d,%Y')[1]"2001-04-26">as.Date('22JUN01',format='%d%b%y')#%yissystem-specific;usewithcaution[1]"2001-06-22"Internally,
Date objects are stored as the number of days since January 1, 1970, using negative numbers for earlier dates. The
as.numericfunction can be used to convert a
Date object to its internal form.To extract the components of the dates, the
weekdays,
months,
days or
quartersfunctions can be used. For example, to find the day of the week on which some famous statisticians were born, we can look at the result of the
weekdays function:
>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)tukeyfishercramerkendall"Wednesday""Monday""Monday""Friday"For an alternative way of extracting pieces of a day, and for informationon possible output formats for
Date objects, see Section.The
chron function converts dates and times to
chron objects.The dates and times are provided to the
chron function as separate values,so some preprocessing may be necessary to prepare input date/times for the
chron function. When using character values, the default format for dates is the decimal month valuefollowed by the decimal day value followed by the year, using the slash as a separator. Alternative formats can be provided by using the codes shown in Table
| Format codes for dates |
| Code | Value |
| m | Month (decimal number) |
| d | Day of the month (decimal number) |
| y | Year (4 digit) |
| mon | Month (abbreviated) |
| month | Month (full name) |
| Format codes for times |
| Code | Value |
| h | Hour |
| m | Minute |
| s | Second |
Alternatively, dates can be specified by a numericvalue, representing the number of days since January 1, 1970. To input dates storedas the day of the year, the
origin= argument can be used to interpret numeric dates relative to a different date. The default format for times consists of the hour, minutes and seconds, separated by colons. Alternative formats can use the codes in Table.Often the first task when using the
chron library is to break apartthe date and times if they are stored together. In the following example, the
strsplit function is used to break apart the string.
>dtimes=c("2002-06-0912:45:40","2003-01-2909:30:40",+"2002-09-0416:45:40","2002-11-1320:00:40",+"2002-07-0717:30:40")>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](02-06-0912:45:40)(03-01-2909:30:40)(02-09-0416:45:40)[4](02-11-1320:00:40)(02-07-0717:30:40)Chron values are stored internally as the fractional number of days fromJanuary 1, 1970. The
as.numericfunction can be used to access the internal values.For information on formatting
chron objects for output, see SectionPOSIX represents a portable operating system interface, primarily for UNIXsystems, but available on other operating systems as well. Dates stored inthe POSIX format are date/time values (like dates with the
chron library),but also allow modification of time zones. Unlike the
chron library,which stores times as fractions of days, the POSIX date classes store times tothe nearestsecond, so they provide a moreaccurate representation of times.There are two POSIX date/time classes, which differ in the way that the valuesare stored internally. The
POSIXct class stores date/time values as thenumber of seconds since January 1, 1970, while the
POSIXlt classstores them as a list with elements for second, minute, hour, day, month, and year,among others. Unless you need the list nature of the
POSIXlt class,the
POSIXct class is the usual choice for storing dates in R.The default input format for POSIX dates consists of the year, followed by the month and day, separated by slashes or dashes; for date/time values, the datemay be followed by white space and a time in the form hour:minutes:seconds or hour:minutes; thus,the following are examples of valid POSIX date or date/time inputs:
1915/6/162005-06-2411:251990/2/1712:20:05
If the input times correspond to one of these formats, as.POSIXct canbe called directly:
>dts=c("2005-10-2118:47:22","2005-12-2416:39:58",+"2005-10-2807:30:05PDT")>as.POSIXlt(dts)[1]"2005-10-2118:47:22""2005-12-2416:39:58"[3]"2005-10-2807:30:05"If your input date/times are stored as the number of seconds from January 1, 1970, you can create
POSIX date values by assigning the appropriate classdirectly to those values. Since most date manipulation functions refer to the
POSIXt psuedo-class, be sure to include it as the first member of the class attribute.
>dts=c(1127056501,1104295502,1129233601,1113547501,+1119826801,1132519502,1125298801,1113289201)>mydates=dts>class(mydates)=c('POSIXt','POSIXct')>mydates[1]"2005-09-1808:15:01PDT""2004-12-2820:45:02PST"[3]"2005-10-1313:00:01PDT""2005-04-1423:45:01PDT"[5]"2005-06-2616:00:01PDT""2005-11-2012:45:02PST"[7]"2005-08-2900:00:01PDT""2005-04-1200:00:01PDT"Conversions like this can be done more succinctly using the
structure function:
>mydates=structure(dts,class=c('POSIXt','POSIXct')) | Code | Meaning | Code | Meaning |
| %a | Abbreviated weekday | %A | Full weekday |
| %b | Abbreviated month | %B | Full month |
| %c | Locale-specific date and time | %d | Decimal date |
| %H | Decimal hours (24 hour) | %I | Decimal hours (12 hour) |
| %j | Decimal day of the year | %m | Decimal month |
| %M | Decimal minute | %p | Locale-specific AM/PM |
| %S | Decimal second | %U | Decimal week of the year (starting on Sunday) |
| %w | Decimal Weekday (0=Sunday) | %W | Decimal week of the year (starting on Monday) |
| %x | Locale-specific Date | %X | Locale-specific Time |
| %y | 2-digit year | %Y | 4-digit year |
| %z | Offset from GMT | %Z | Time zone (character) |
The POSIX date/time classes take advantage of the POSIX date/time implementationof your operating system, allowing dates and times in R to be manipulated in the same way they would in, for example a C program. The two most important functionsin this regard are
strptime, for inputting dates, and
strftime, for formatting dates for output. Both of these functions use a variety of formatting codes, some of which are listed in Table, to specify the way dates are read or printed. For example,dates in many logfiles are printed in a format like "
16/Oct/2005:07:51:00".To create a
POSIXct date from a date in this format, the following call to
strptime could be used:
>mydate=strptime('16/Oct/2005:07:51:00',format='%d/%b/%Y:%H:%M:%S')[1]"2005-10-1607:51:00" Note that non-format characters (like the slashes) are interpretedliterally.When using
strptime, an optional time zone can be specifiedwith the
tz= option.Another way to create POSIX dates is to pass the individual components of the time to the ISOdate function. Thus, the first date/time value in the previous example could also be created with a call to
ISOdate;
>ISOdate(2005,10,21,18,47,22,tz="PDT")[1]"2005-10-2118:47:22PDT"
For formatting dates for output, the
format function will recognizethe type of your input date, and perform any necessary conversions before calling
strftime, so
strftime rarely needs to be called directly. For example, to print a date/time value in an extended format, we could use:
>thedate=ISOdate(2005,10,21,18,47,22,tz="PDT")>format(thedate,'%A,%B%d,%Y%H:%M:%S')[1]"Friday,October21,200518:47:22"
When using POSIX dates, the optional
usetz=TRUE argumentto the
format functioncan be specified to indicate that the time zone should be displayed. Additionally,
as.POSIXlt and
as.POSIXct can alsoaccept
Date or
chron objects, so they can be input as described in the previous sections and converted as needed. Conversion betweenthe two POSIX forms is also possible.The individual components of a POSIX date/time object can be extracted by first converting to
POSIXlt if necessary, and then accessing the components directly:
>mydate=as.POSIXlt('2005-4-197:01:00')>names(mydate)[1]"sec""min""hour""mday""mon""year"[7]"wday""yday""isdst">mydate$mday[1]19Many of the statistical summary functions, like
mean,
min,
max, etc are able to transparently handle date objects. For example,consider the releasedates of various versions or R from 1.0 to 2.0:
>rdates=scan(what="")1:1.029Feb20003:1.115Jun20005:1.215Dec20007:1.322Jun20019:1.419Dec200111:1.529Apr200213:1.61Oct200215:1.716Apr200317:1.88Oct200319:1.912Apr200421:2.04Oct200423:Read22items>rdates=as.data.frame(matrix(rdates,ncol=2,byrow=TRUE))>rdates[,2]=as.Date(rdates[,2],format='%d%b%Y')>names(rdates)=c("Release","Date")>rdatesReleaseDate11.02000-02-2921.12000-06-1531.22000-12-1541.32001-06-2251.42001-12-1961.52002-04-2971.62002-10-0181.72003-04-1691.82003-10-08101.92004-04-12112.02004-10-04Once the dates are properly read into R, a variety of calculations canbe performed:
>mean(rdates$Date)[1]"2002-05-19">range(rdates$Date)[1]"2000-02-29""2004-10-04">rdates$Date[11]-rdates$Date[1]Timedifferenceof1679days
If two times (using any of the date or date/time classes) are subtracted,R will return the results in the form of a time difference, which representsa
difftime object. For example, New York City experienceda major blackout on July 13, 1997, and another on August 14, 2003. To calculatethe time interval between the two blackouts, we can simply subtract the twodates, using any of the classes that have been introduced:
>b1=ISOdate(1977,7,13)>b2=ISOdate(2003,8,14)>b2-b1Timedifferenceof9528days
If an alternative unit of time was desired, the
difftimefunction could be called, using the optional
units= argument can be usedwith any of the following values: "
auto", "
secs", "
mins", "
hours", "
days", or "
weeks".So to see the difference between blackouts in terms of weeks, we can use:
>difftime(b2,b1,units='weeks')Timedifferenceof1361.143weeks
Although
difftime values are displayed with their units, theycan be manipulated like ordinary numeric variables; arithmetic performed with thesevalues will retain the original units.The
by= argument to the
seq function can be specified either asa
difftime value, or in any unitsof time that the
difftime function accepts, making it very easy to generate sequences of dates. For example, to generate a vector of ten dates, starting on July 4, 1976 with an interval of oneday between them, we could use:
>seq(as.Date('1976-7-4'),by='days',length=10)[1]"1976-07-04""1976-07-05""1976-07-06""1976-07-07""1976-07-08"[6]"1976-07-09""1976-07-10""1976-07-11""1976-07-12""1976-07-13" All the date classes except for
chron will accept an integerbefore the interval provided as a
by= argument. We could create a sequence of dates separated by two weeks from June 1, 2000 to August 1, 2000 asfollows:
>seq(as.Date('2000-6-1'),to=as.Date('2000-8-1'),by='2weeks')[1]"2000-06-01""2000-06-15""2000-06-29""2000-07-13""2000-07-27" The
cut function also understands units of
days,
weeks,
months, and
years, making it very easy to create factors grouped by these units. See Section for details.Format codes can also be used to extract parts of dates, similar tothe
weekdays and other functions described in Section.We could look at the distribution of weekdays for the R release dates as follows:
>table(format(rdates$Date,'%A'))FridayMondayThursdayTuesdayWednesday23123
This same technique can be used to convert dates to factors. For example, to create a factor based on the release dates broken down by yearswe could use:
>fdate=factor(format(rdates$Date,'%Y'))>fdate[1]20002000200020012001200220022003200320042004Levels:20002001200220032004
>cut(thetimes,"year")[1]0203020202Levels:02<03
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