| as.POSIX* {base} | R Documentation |
Functions to manipulate objects of classes "POSIXlt" and
"POSIXct" representing calendar dates and times.
as.POSIXct(x, tz = "") as.POSIXlt(x, tz = "") ## S3 method for class 'POSIXlt': as.numeric(x)
x |
An object to be converted. |
tz |
A timezone specification to be used for the conversion,
if one is required. System-specific, but ""
is the current timezone, and "GMT" is UTC
(Coordinated Universal Time, in French). |
The as.POSIX* functions convert an object to one of the two
classes used to represent date/times (calendar dates plus time to the
nearest second). They can convert a wide variety of objects,
including objects of the other class and of classes "Date",
"date" (from package date or
survival), "chron" and
"dates" (from package chron) to these
classes. Dates without times are treated as being at midnight UTC.
They can also convert character strings of the formats
"2001-02-03" and "2001/02/03" optionally followed by
white space and a time in the format "14:52" or
"14:52:03". (Formats such as "01/02/03" are ambiguous
but can be converted via a format specification by
strptime.) (As from R 2.4.0 fractional seconds can be
converted.)
Logical NAs can be converted to either of the classes, but no
other logical vectors can be.
The as.numeric method converts "POSIXlt" objects to
"POSIXct".
If you are given a numeric time as the number of seconds since an epoch, see the examples.
Where OSes describe their valid timezones can be obscure. The help
for tzset (or _tzset on Windows) can be helpful, but it
can also be inaccurate. There is a cumbersome POSIX specification,
(listed under environment variable TZ at
http://www.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap08.html),
which is often at least partially supported, but there may be other more
user-friendly ways to specify timezones.
Windows documents a specification of the form GST-1GDT, but
seems always to apply the US rules for changing to/from DST with such a
specification. (There appears to be no documented way to apply
timezones using other DST rules except to switch to the timezone,
e.g. from the Control Panel.) So valid time zone values are of the
forms EST5EDT for a timezone 5 hours west of GMT called
EST when daylight saving is in force and EDT when it is
not, HST10 for ‘Hawaii-Aleutian Standard Time’ (which has no
DST), and CST-9:30 for the Northern Territory of Australia.
The abbreviations have to be exactly 3 letters, and have no meaning
other than as labels on the output.
as.POSIXct and as.POSIXlt return an object of the
appropriate class. If tz was specified, as.POSIXlt
will give an appropriate "tzone" attribute.
If you want to extract specific aspects of a time (such as the day of
the week) just convert it to class "POSIXlt" and extract the
relevant component(s) of the list, or if you want a character
representation (such as a named day of the week) use
format.POSIXlt or format.POSIXct.
If a timezone is needed and that specified is invalid on your system, what happens is system-specific but it will probably be ignored.
DateTimeClasses for details of the classes;
strptime for conversion to and from character
representations.
(z <- Sys.time()) # the current datetime, as class "POSIXct"
unclass(z) # a large integer
floor(unclass(z)/86400) # the number of days since 1970-01-01
(z <- as.POSIXlt(Sys.time())) # the current datetime, as class "POSIXlt"
unlist(unclass(z)) # a list shown as a named vector
## suppose we have a time in seconds since 1960-01-01 00:00:00 GMT
z <- 1472562988
# two ways to convert this
ISOdatetime(1960,1,1,0,0,0) + z # late August 2006
strptime("1960-01-01", "%Y-%m-%d", tz="GMT") + z
as.POSIXlt(Sys.time(), "GMT") # the current time in GMT
as.POSIXlt(Sys.time(), "EST5EDT") # the current time in New York
as.POSIXlt(Sys.time(), "HST10") # the current time in Hawaii
as.POSIXlt(Sys.time(), "CST-9:30") # the current time in Darwin