help

$ date --help
Usage: date [OPTION]... [+FORMAT]
  or:  date [-u|--utc|--universal] [MMDDhhmm[[CC]YY][.ss]]
Display the current time in the given FORMAT, or set the system date.
Mandatory arguments to long options are mandatory for short options too.
  -d, --date=STRING         display time described by STRING, not 'now'
  -f, --file=DATEFILE       like --date once for each line of DATEFILE
  -I[TIMESPEC], --iso-8601[=TIMESPEC]  output date/time in ISO 8601 format.
                            TIMESPEC='date' for date only (the default),
                            'hours', 'minutes', 'seconds', or 'ns' for date
                            and time to the indicated precision.
  -r, --reference=FILE      display the last modification time of FILE
  -R, --rfc-2822            output date and time in RFC 2822 format.
                            Example: Mon, 07 Aug 2006 12:34:56 -0600
      --rfc-3339=TIMESPEC   output date and time in RFC 3339 format.
                            TIMESPEC='date', 'seconds', or 'ns' for
                            date and time to the indicated precision.
                            Date and time components are separated by
                            a single space: 2006-08-07 12:34:56-06:00
  -s, --set=STRING          set time described by STRING
  -u, --utc, --universal    print or set Coordinated Universal Time (UTC)
      --help     display this help and exit
      --version  output version information and exit
FORMAT controls the output.  Interpreted sequences are:
  %%   a literal %
  %a   locale's abbreviated weekday name (e.g., Sun)
  %A   locale's full weekday name (e.g., Sunday)
  %b   locale's abbreviated month name (e.g., Jan)
  %B   locale's full month name (e.g., January)
  %c   locale's date and time (e.g., Thu Mar  3 23:05:25 2005)
  %C   century; like %Y, except omit last two digits (e.g., 20)
  %d   day of month (e.g., 01)
  %D   date; same as %m/%d/%y
  %e   day of month, space padded; same as %_d
  %F   full date; same as %Y-%m-%d
  %g   last two digits of year of ISO week number (see %G)
  %G   year of ISO week number (see %V); normally useful only with %V
  %h   same as %b
  %H   hour (00..23)
  %I   hour (01..12)
  %j   day of year (001..366)
  %k   hour, space padded ( 0..23); same as %_H
  %l   hour, space padded ( 1..12); same as %_I
  %m   month (01..12)
  %M   minute (00..59)
  %n   a newline
  %N   nanoseconds (000000000..999999999)
  %p   locale's equivalent of either AM or PM; blank if not known
  %P   like %p, but lower case
  %r   locale's 12-hour clock time (e.g., 11:11:04 PM)
  %R   24-hour hour and minute; same as %H:%M
  %s   seconds since 1970-01-01 00:00:00 UTC
  %S   second (00..60)
  %t   a tab
  %T   time; same as %H:%M:%S
  %u   day of week (1..7); 1 is Monday
  %U   week number of year, with Sunday as first day of week (00..53)
  %V   ISO week number, with Monday as first day of week (01..53)
  %w   day of week (0..6); 0 is Sunday
  %W   week number of year, with Monday as first day of week (00..53)
  %x   locale's date representation (e.g., 12/31/99)
  %X   locale's time representation (e.g., 23:13:48)
  %y   last two digits of year (00..99)
  %Y   year
  %z   +hhmm numeric time zone (e.g., -0400)
  %:z  +hh:mm numeric time zone (e.g., -04:00)
  %::z  +hh:mm:ss numeric time zone (e.g., -04:00:00)
  %:::z  numeric time zone with : to necessary precision (e.g., -04, +05:30)
  %Z   alphabetic time zone abbreviation (e.g., EDT)
By default, date pads numeric fields with zeroes.
The following optional flags may follow '%':
  -  (hyphen) do not pad the field
  _  (underscore) pad with spaces
  0  (zero) pad with zeros
  ^  use upper case if possible
  #  use opposite case if possible
After any flags comes an optional field width, as a decimal number;
then an optional modifier, which is either
E to use the locale's alternate representations if available, or
O to use the locale's alternate numeric symbols if available.
Examples:
Convert seconds since the epoch (1970-01-01 UTC) to a date
  $ date --date='@2147483647'
Show the time on the west coast of the US (use tzselect(1) to find TZ)
  $ TZ='America/Los_Angeles' date
Show the local time for 9AM next Friday on the west coast of the US
  $ date --date='TZ="America/Los_Angeles" 09:00 next Fri'
GNU coreutils online help: <http://www.gnu.org/software/coreutils/>
For complete documentation, run: info coreutils 'date invocation'

Man

$ man date
DATE(1)                                                  User Commands                                                  DATE(1)
NAME
       date - print or set the system date and time
SYNOPSIS
       date [OPTION]... [+FORMAT]
       date [-u|--utc|--universal] [MMDDhhmm[[CC]YY][.ss]]
DESCRIPTION
       Display the current time in the given FORMAT, or set the system date.
       Mandatory arguments to long options are mandatory for short options too.
       -d, --date=STRING
              display time described by STRING, not 'now'
       -f, --file=DATEFILE
              like --date once for each line of DATEFILE
       -I[TIMESPEC], --iso-8601[=TIMESPEC]
              output date/time in ISO 8601 format.  TIMESPEC='date' for date only (the default), 'hours', 'minutes', 'seconds',
              or 'ns' for date and time to the indicated precision.
       -r, --reference=FILE
              display the last modification time of FILE
       -R, --rfc-2822
              output date and time in RFC 2822 format.  Example: Mon, 07 Aug 2006 12:34:56 -0600
       --rfc-3339=TIMESPEC
              output date and time in RFC 3339 format.  TIMESPEC='date', 'seconds', or 'ns' for date and time to the  indicated
              precision.  Date and time components are separated by a single space: 2006-08-07 12:34:56-06:00
       -s, --set=STRING
              set time described by STRING
       -u, --utc, --universal
              print or set Coordinated Universal Time (UTC)
       --help display this help and exit
       --version
              output version information and exit
       FORMAT controls the output.  Interpreted sequences are:
       %%     a literal %
       %a     locale's abbreviated weekday name (e.g., Sun)
       %A     locale's full weekday name (e.g., Sunday)
       %b     locale's abbreviated month name (e.g., Jan)
       %B     locale's full month name (e.g., January)
       %c     locale's date and time (e.g., Thu Mar  3 23:05:25 2005)
       %C     century; like %Y, except omit last two digits (e.g., 20)
       %d     day of month (e.g., 01)
       %D     date; same as %m/%d/%y
       %e     day of month, space padded; same as %_d
       %F     full date; same as %Y-%m-%d
       %g     last two digits of year of ISO week number (see %G)
       %G     year of ISO week number (see %V); normally useful only with %V
       %h     same as %b
       %H     hour (00..23)
       %I     hour (01..12)
       %j     day of year (001..366)
       %k     hour, space padded ( 0..23); same as %_H
       %l     hour, space padded ( 1..12); same as %_I
       %m     month (01..12)
       %M     minute (00..59)
       %n     a newline
       %N     nanoseconds (000000000..999999999)
       %p     locale's equivalent of either AM or PM; blank if not known
       %P     like %p, but lower case
       %r     locale's 12-hour clock time (e.g., 11:11:04 PM)
       %R     24-hour hour and minute; same as %H:%M
       %s     seconds since 1970-01-01 00:00:00 UTC
       %S     second (00..60)
       %t     a tab
       %T     time; same as %H:%M:%S
       %u     day of week (1..7); 1 is Monday
       %U     week number of year, with Sunday as first day of week (00..53)
       %V     ISO week number, with Monday as first day of week (01..53)
       %w     day of week (0..6); 0 is Sunday
       %W     week number of year, with Monday as first day of week (00..53)
       %x     locale's date representation (e.g., 12/31/99)
       %X     locale's time representation (e.g., 23:13:48)
       %y     last two digits of year (00..99)
       %Y     year
       %z     +hhmm numeric time zone (e.g., -0400)
       %:z    +hh:mm numeric time zone (e.g., -04:00)
       %::z   +hh:mm:ss numeric time zone (e.g., -04:00:00)
       %:::z  numeric time zone with : to necessary precision (e.g., -04, +05:30)
       %Z     alphabetic time zone abbreviation (e.g., EDT)
       By default, date pads numeric fields with zeroes.  The following optional flags may follow '%':
       -      (hyphen) do not pad the field
       _      (underscore) pad with spaces
       0      (zero) pad with zeros
       ^      use upper case if possible
       #      use opposite case if possible
       After  any flags comes an optional field width, as a decimal number; then an optional modifier, which is either E to use
       the locale's alternate representations if available, or O to use the locale's alternate numeric symbols if available.
ENVIRONMENT
       TZ     Specifies the timezone, unless overridden by command line parameters.  If neither is specified, the setting  from
              /etc/localtime is used.
EXAMPLES
       Convert seconds since the epoch (1970-01-01 UTC) to a date
              $ date --date='@2147483647'
       Show the time on the west coast of the US (use tzselect(1) to find TZ)
              $ TZ='America/Los_Angeles' date
       Show the local time for 9AM next Friday on the west coast of the US
              $ date --date='TZ="America/Los_Angeles" 09:00 next Fri'
       GNU coreutils online help: <http://www.gnu.org/software/coreutils/> Report date translation bugs to <http://translation‐
       project.org/team/>
DATE STRING
       The --date=STRING is a mostly free format human readable date string such as  "Sun,  29  Feb  2004  16:21:42  -0800"  or
       "2004-02-29  16:21:42"  or even "next Thursday".  A date string may contain items indicating calendar date, time of day,
       time zone, day of week, relative time, relative date, and numbers.  An empty string indicates the beginning of the  day.
       The date string format is more complex than is easily documented here but is fully described in the info documentation.
AUTHOR
       Written by David MacKenzie.
COPYRIGHT
       Copyright    ©   2013   Free   Software   Foundation,   Inc.    License   GPLv3+:   GNU   GPL   version   3   or   later
       <http://gnu.org/licenses/gpl.html>.
       This is free software: you are free to change and redistribute it.  There is NO WARRANTY, to  the  extent  permitted  by
       law.
SEE ALSO
       The full documentation for date is maintained as a Texinfo manual.  If the info and date programs are properly installed
       at your site, the command
              info coreutils 'date invocation'
       should give you access to the complete manual.
GNU coreutils 8.22                                       November 2016                                                  DATE(1)

Example

$ date
Mon Apr  1 11:52:12 CST 2019
$ date +"%Y-%m-%d"
2019-01-02