Common shell date methods

Obtain the time string in Linux
Command date
# Output the current time of 23 days in the format of yyyymmdd
$ Date + % Y % m % d -- date = '23 days ago'

$ Date-u
Thu Sep 28 09:32:04 UTC 2006

$ Date-R
Thu, 28 Sep 2006 17:32:28 + 0800

# Testing one thousandth of a second
$ Date + '% Y % m % d % H: % m: % S. % n'; date + '% Y % m % d % H: % m: % S. % n'; date + '% Y % m % d % H: % m: % S. % n'; date + '% Y % m % d % H: % m: % S. % N'
20060928 17:44:20. 906805000
20060928 17:44:20. 909188000
20060928 17:44:20. 911535000
20060928 17:44:20. 913886000

Date reference
$ 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.

-D, -- date = string display time described by string, not 'now'
# Such as 'n' days ago | 1 month ago | N years ago'
-F, -- file = datefile like -- date once for each line of datefile
-Itimespec, -- iso-8601 [= timespec] Output date/time in ISO 8601 format.
Timespec = 'date' for date only,
'Urs', 'minutes ', or 'seconds' for date and
Time to the indicated precision.
-- Iso-8601 without timespec defaults to 'date '.
-R, -- reference = file display the last modification time of File
-R, -- rfc-2822 output RFC-2822 compliant date string
-S, -- set = string set time described by string
-U, -- UTC, -- Universal print or set Coordinated Universal Time
-- Help display this help and exit
-- Version output version information and exit

Format controls the output. The only valid option for the second form
Specifies Coordinated Universal Time. interpreted sequences are:

% Output % sign a literal %
% A the abbreviation of the week for the current domain locale's abbreviated weekday name (Sun .. SAT)
% A indicates that locale's full weekday name and variable length (Sunday .. Saturday) are all written for the current domain in the week)
% B the abbreviation of the month in the current domain locale's abbreviated month name (Jan... dec)
% B full month name of the current domain locale's full month name, variable length (January... December)
% C default time format of the current domain locale's date and time (Sat Nov 04 12:02:33 est 1989)
% C n Centennial Century (year divided by 100 and truncated to an integer) [00-99]
Day of month (01 .. 31) of % d)
% D short time format date (mm/DD/yy)
% E short format day of month, blank padded (1 .. 31)
% F file time format same as % Y-% m-% d
% G the 2-digit year corresponding to the % v week number
% G the 4-digit year corresponding to the % v week number
% H same as % B
% H hour in the 24-hour format (00 .. 23)
% I hour of 12 hour (01 .. 12)
% J day of year (001 .. 366) of the year)
% K short format hour (0 .. 23)
% L hour (1 .. 12) in 12-hour short format)
% M double-digit month (01 .. 12)
% M dual-bit minute (00 .. 59)
% N wrap a newline
% N 000000000 s nanoseconds (999999999)
% P upper-case current domain upper-afternoon indication locale's upper case am or PM indicator (blank in upper Locales)
% P upper-afternoon indication of the current domain in lower case locale's lower case am or PM indicator (blank in lower Locales)
% R 12-hour time representation (hour: minute: Second, double-digit) time, 12-hour (HH: mm: ss [AP] m)
% R 24-hour time representation (hour: minute, double digit) time, 24-hour (HH: mm)
% S seconds from basic time 00:00:00 to current time seconds since '00: 00: 00 UTC '(a GNU extension)
% S dual-bit second (00 .. 60); the 60 is necessary to accommodate a leap second
% T horizontal tab (Tab) a horizontal Tab
% T 24-hour time indicates time, 24-hour (HH: mm: SS)
% U indicates the day of the week (from Monday to Monday) of Week (1 .. 7); 1 represents Monday
% U week of the year Sunday is the start week number of year with Sunday as first day of week (00 .. 53)
% V The week Monday of the year is the start week number of year with Monday as first day of week (01 .. 53)
% W the day of the week Sunday starts 0-6 days of week (0 .. 6); 0 represents Sunday
% W week of the year Monday is the start week number of year with Monday as first day of week (00 .. 53)
% X local Date Format locale's date representation (mm/DD/yy)
% X local time format locale's time representation (% H: % m: % s)
Last two digits of year (00 .. 99) of % Y)
% Y year Year (1970 ...)
% Z RFC-2822 Standard Time Format representation of the domain RFC-2822 style numeric timezone (-0500) (a nonstandard extension)
% Z Time Zone (e.g., EDT), or nothing if no time zone is determinable

By default, date pads numeric fields with zeroes. GNU date recognizes
The following modifiers between '%' and a numeric directive.

'-' (Hyphen) do not pad the field
'_' (Underscore) pad the field with spaces

Http://websir.blogspot.com/2006/09/shelldate.html

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