freebsd-src/bin/date/date.1

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.\"     @(#)date.1	8.3 (Berkeley) 4/28/95
.\" $FreeBSD$
.\"
.Dd June 2, 2007
.Dt DATE 1
.Os
.Sh NAME
.Nm date
.Nd display or set date and time
.Sh SYNOPSIS
.Nm
.Op Fl ju
.Op Fl r Ar seconds
.Oo
.Fl v
.Sm off
.Op Cm + | -
.Ar val Op Ar ymwdHMS
.Sm on
.Oc
.Ar ...
.Op Cm + Ns Ar output_fmt
.Nm
.Op Fl jnu
.Sm off
.Op Oo Oo Oo Oo Ar cc Oc Ar yy Oc Ar mm Oc Ar dd Oc Ar HH
.Ar MM Op Ar .ss
.Sm on
.Nm
.Op Fl jnu
.Fl f Ar input_fmt new_date
.Op Cm + Ns Ar output_fmt
.Nm
.Op Fl d Ar dst
.Op Fl t Ar minutes_west
.Sh DESCRIPTION
When invoked without arguments, the
.Nm
utility displays the current date and time.
Otherwise, depending on the options specified,
.Nm
will set the date and time or print it in a user-defined way.
.Pp
The
.Nm
utility displays the date and time read from the kernel clock.
When used to set the date and time,
both the kernel clock and the hardware clock are updated.
.Pp
Only the superuser may set the date,
and if the system securelevel (see
.Xr securelevel 7 )
is greater than 1,
the time may not be changed by more than 1 second.
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl d Ar dst
Set the kernel's value for daylight saving time.
If
.Ar dst
is non-zero, future calls
to
.Xr gettimeofday 2
will return a non-zero for
.Fa tz_dsttime .
.It Fl f
Use
.Ar input_fmt
as the format string to parse the
.Ar new_date
provided rather than using the default
.Sm off
.Oo Oo Oo Oo Oo
.Ar cc Oc
.Ar yy Oc
.Ar mm Oc
.Ar dd Oc
.Ar HH
.Oc Ar MM Op Ar .ss
.Sm on
format.
Parsing is done using
.Xr strptime 3 .
.It Fl j
Do not try to set the date.
This allows you to use the
.Fl f
flag in addition to the
.Cm +
option to convert one date format to another.
.It Fl n
By default, if the
.Xr timed 8
daemon is running,
.Nm
sets the time on all of the machines in the local group.
The
.Fl n
option suppresses this behavior and causes the time to be set only on the
current machine.
.It Fl r Ar seconds
Print the date and time represented by
.Ar seconds ,
where
.Ar seconds
is the number of seconds since the Epoch
(00:00:00 UTC, January 1, 1970;
see
.Xr time 3 ) ,
and can be specified in decimal, octal, or hex.
.It Fl t Ar minutes_west
Set the system's value for minutes west of
.Tn GMT .
.Ar minutes_west
specifies the number of minutes returned in
.Fa tz_minuteswest
by future calls to
.Xr gettimeofday 2 .
.It Fl u
Display or set the date in
.Tn UTC
(Coordinated Universal) time.
.It Fl v
Adjust (i.e., take the current date and display the result of the
adjustment; not actually set the date) the second, minute, hour, month
day, week day, month or year according to
.Ar val .
If
.Ar val
is preceded with a plus or minus sign,
the date is adjusted forwards or backwards according to the remaining string,
otherwise the relevant part of the date is set.
The date can be adjusted as many times as required using these flags.
Flags are processed in the order given.
.Pp
When setting values
(rather than adjusting them),
seconds are in the range 0-59, minutes are in the range 0-59, hours are
in the range 0-23, month days are in the range 1-31, week days are in the
range 0-6 (Sun-Sat),
months are in the range 1-12 (Jan-Dec)
and years are in the range 80-38 or 1980-2038.
.Pp
If
.Ar val
is numeric, one of either
.Ar y ,
.Ar m ,
.Ar w ,
.Ar d ,
.Ar H ,
.Ar M
or
.Ar S
must be used to specify which part of the date is to be adjusted.
.Pp
The week day or month may be specified using a name rather than a
number.
If a name is used with the plus
(or minus)
sign, the date will be put forwards
(or backwards)
to the next
(previous)
date that matches the given week day or month.
This will not adjust the date,
if the given week day or month is the same as the current one.
.Pp
When a date is adjusted to a specific value or in units greater than hours,
daylight savings time considerations are ignored.
Adjustments in units of hours or less honor daylight saving time.
So, assuming the current date is March 26, 0:30 and that the DST adjustment
means that the clock goes forward at 01:00 to 02:00, using
.Fl v No +1H
will adjust the date to March 26, 2:30.
Likewise, if the date is October 29, 0:30 and the DST adjustment means that
the clock goes back at 02:00 to 01:00, using
.Fl v No +3H
will be necessary to reach October 29, 2:30.
.Pp
When the date is adjusted to a specific value that does not actually exist
(for example March 26, 1:30 BST 2000 in the Europe/London timezone),
the date will be silently adjusted forwards in units of one hour until it
reaches a valid time.
When the date is adjusted to a specific value that occurs twice
(for example October 29, 1:30 2000),
the resulting timezone will be set so that the date matches the earlier of
the two times.
.Pp
Adjusting the date by months is inherently ambiguous because
a month is a unit of variable length depending on the current date.
This kind of date adjustment is applied in the most intuitive way.
First of all,
.Nm
tries to preserve the day of the month.
If it is impossible because the target month is shorter than the present one,
the last day of the target month will be the result.
For example, using
.Fl v No +1m
on May 31 will adjust the date to June 30, while using the same option
on January 30 will result in the date adjusted to the last day of February.
This approach is also believed to make the most sense for shell scripting.
Nevertheless, be aware that going forth and back by the same number of
months may take you to a different date.
.Pp
Refer to the examples below for further details.
.El
.Pp
An operand with a leading plus
.Pq Sq +
sign signals a user-defined format string
which specifies the format in which to display the date and time.
The format string may contain any of the conversion specifications
described in the
.Xr strftime 3
manual page, as well as any arbitrary text.
A newline
.Pq Ql \en
character is always output after the characters specified by
the format string.
The format string for the default display is
.Dq +%+ .
.Pp
If an operand does not have a leading plus sign, it is interpreted as
a value for setting the system's notion of the current date and time.
The canonical representation for setting the date and time is:
.Pp
.Bl -tag -width Ds -compact -offset indent
.It Ar cc
Century
(either 19 or 20)
prepended to the abbreviated year.
.It Ar yy
Year in abbreviated form
(e.g., 89 for 1989, 06 for 2006).
.It Ar mm
Numeric month, a number from 1 to 12.
.It Ar dd
Day, a number from 1 to 31.
.It Ar HH
Hour, a number from 0 to 23.
.It Ar MM
Minutes, a number from 0 to 59.
.It Ar ss
Seconds, a number from 0 to 61
(59 plus a maximum of two leap seconds).
.El
.Pp
Everything but the minutes is optional.
.Pp
Time changes for Daylight Saving Time, standard time, leap seconds,
and leap years are handled automatically.
.Sh ENVIRONMENT
The following environment variables affect the execution of
.Nm :
.Bl -tag -width Ds
.It Ev TZ
The timezone to use when displaying dates.
The normal format is a pathname relative to
.Pa /usr/share/zoneinfo .
For example, the command
.Dq TZ=America/Los_Angeles date
displays the current time in California.
See
.Xr environ 7
for more information.
.El
.Sh FILES
.Bl -tag -width /var/log/messages -compact
.It Pa /var/log/wtmp
record of date resets and time changes
.It Pa /var/log/messages
record of the user setting the time
.El
.Sh EXIT STATUS
The
.Nm
utility exits 0 on success, 1 if unable to set the date, and 2
if able to set the local date, but unable to set it globally.
.Sh EXAMPLES
The command:
.Pp
.Dl "date ""+DATE: %Y-%m-%d%nTIME: %H:%M:%S"""
.Pp
will display:
.Bd -literal -offset indent
DATE: 1987-11-21
TIME: 13:36:16
.Ed
.Pp
In the Europe/London timezone, the command:
.Pp
.Dl "date -v1m -v+1y"
.Pp
will display:
.Pp
.Dl "Sun Jan  4 04:15:24 GMT 1998"
.Pp
where it is currently
.Li "Mon Aug  4 04:15:24 BST 1997" .
.Pp
The command:
.Pp
.Dl "date -v1d -v3m -v0y -v-1d"
.Pp
will display the last day of February in the year 2000:
.Pp
.Dl "Tue Feb 29 03:18:00 GMT 2000"
.Pp
So will do the command:
.Pp
.Dl "date -v30d -v3m -v0y -v-1m"
.Pp
because there is no such date as the 30th of February.
.Pp
The command:
.Pp
.Dl "date -v1d -v+1m -v-1d -v-fri"
.Pp
will display the last Friday of the month:
.Pp
.Dl "Fri Aug 29 04:31:11 BST 1997"
.Pp
where it is currently
.Li "Mon Aug  4 04:31:11 BST 1997" .
.Pp
The command:
.Pp
.Dl "date 8506131627"
.Pp
sets the date to
.Dq Li "June 13, 1985, 4:27 PM" .
.Pp
.Dl "date ""+%Y%m%d%H%M.%S"""
.Pp
may be used on one machine to print out the date
suitable for setting on another.
.Qq ( Li "+%m%d%H%M%Y.%S"
for use on
.Tn Linux . )
.Pp
The command:
.Pp
.Dl "date 1432"
.Pp
sets the time to
.Li "2:32 PM" ,
without modifying the date.
.Pp
Finally the command:
.Pp
.Dl "date -j -f ""%a %b %d %T %Z %Y"" ""`date`"" ""+%s"""
.Pp
can be used to parse the output from
.Nm
and express it in Epoch time.
.Sh DIAGNOSTICS
Occasionally, when
.Xr timed 8
synchronizes the time on many hosts, the setting of a new time value may
require more than a few seconds.
On these occasions,
.Nm
prints:
.Ql Network time being set .
The message
.Ql Communication error with timed
occurs when the communication
between
.Nm
and
.Xr timed 8
fails.
.Sh SEE ALSO
.Xr locale 1 ,
.Xr gettimeofday 2 ,
.Xr strftime 3 ,
.Xr strptime 3 ,
.Xr utmp 5 ,
.Xr timed 8
.Rs
.%T "TSP: The Time Synchronization Protocol for UNIX 4.3BSD"
.%A R. Gusella
.%A S. Zatti
.Re
.Sh STANDARDS
The
.Nm
utility is expected to be compatible with
.St -p1003.2 .
The
.Fl d , f , j , n , r , t ,
and
.Fl v
options are all extensions to the standard.
.Sh HISTORY
A
.Nm
command appeared in
.At v1 .