Rails 中的时间处理
Time、Date、Datetime 的转换和区别
- Time 主要是对底层的C库时间函数的封装。这些函数通常基于UNIX纪元，因此不能表示1970年前的时间。
- Date 是为了弥补 Time 的缺点创建的，它能轻松地处理较早的时间，如达芬奇的生日（1452年4月15日），还能够智能处理历法改革的日期。但它也有缺点，如不能处理达芬奇的出生时间，它只能处理日期。
- DateTime 继承了 Date，旨在比上述两者都好。它可以像 Date 那样表示日期，也可以像 Time 那样表示时间。它通常是表示日期——时间值的推荐方式。
格式化输出
DateTime.parse(Time.now.to_s).strftime('%Y-%m-%d %H:%M:%S').to_s#就是按照2009-5-14 8：42：13的给定格式输出
反向格式化
DateTime.parse(params['start_date']).strftime('%Y-%m-%d %H:%M:%S').to_s
集成多种方式输出
# config/initializers/date_time_formats.rb
Time::DATE_FORMATS.merge!(
:full => '%B %d, %Y at %I:%M %p',
:md => '%m/%d',
:mdy => '%m/%d/%y',
:time => '%I:%M %p'
)
你就可以简单的通过调用
Time.now.to_s(:full)#按照之前定义"May 14, 2009 at 08:39 AM"
满足变化需求的输出。比如，要求是当前年份，不显示年，其他的年才显示
Time::DATE_FORMATS.merge!(
:friendly => lambda { |time|
if time.year == Time.now.year
time.strftime "%b #{time.day.ordinalize}"
else
time.strftime "%b #{time.day.ordinalize}, %Y"
end
}
)
>> Time.now.to_s(:friendly)
=> "May 14th"
>> (Time.now-2.years).to_s(:friendly)
=> "May 14th, 2007"
to_s(format = :default) public
:db format outputs time in UTC; all others output time in local. Uses TimeWithZone’s strftime, so %Z and %z work correctly.
:db # => 2008-12-25 14:35:05
:number # => 20081225143505
:time # => 14:35
:short # => 25 Dec 14:35
:long # => December 25, 2008 14:35
:long_ordinal # => December 25th, 2008 14:35
:rfc822 # => Thu, 25 Dec 2008 14:35:05 +0000
可以使用的参数说明
This is a major reminder, as I constantly have to search for this one whenever it comes to formatting dates and times in Ruby / Rails:
t = Time.now #=> 2007-11-19 08:37:48 -0600
t.strftime("Printed on %m/%d/%Y") #=> "Printed on 11/19/2007"
t.strftime("at %I:%M%p") #=> "at 08:37AM
Year
%Y year with century 2011
%y year without century 11
%C century number (year divided by 100) 20
Month
%B full month name January
%b abbreviated month name Jan
%h same as %b Jan
%m month as number (01-12)
Week
%U week number of the year, Sunday as first day of week (00-53)
%W week number of the year, Monday as first day of week (00-53)
Day
%A full weekday name Wednesday
%a abbreviated weekday name Wed
%d day of the month (01-31)
%e day of the month, single digits preceded by space ( 1-31)
%j day of the year (001-366)
%w weekday as a number, with 0 representing Sunday (0-6)
%u weekday as a number, with 1 representing Monday (1-7)
Time
%H hour (24-hour clock) (00-23)
%k hour (24-hour clock); single digits preceded by space ( 0-23)
%I hour (12-hour clock) (01-12)
%l hour (12-hour clock); single digits preceded by space ( 1-12)
%M minute (00-59)
%S seconds (00-59)
%p either AM or PM AM
%Z timezone name or abbreviation EDT
%z timezone offset from UTC -0400
Summaries
%D date, same as %m/%d/%y 05/16/07
%v date, same as %e-%b-%Y 16-May-2007
%F date, same as %Y-%m-%d 2007-05-16
%R time, 24 hour notation, same as %H:%M 18:06
%T time, 24 hour notation, same as %H:%M:%S 18:06:15
%r time, am/pm notation, same as %I:%M:%S %p 06:06:15 PM
Formatting
%n newline character
%t tab character
%% percent character
Less common formats
%s number of seconds since the Epoch, UTC
%c national date and time representation
%+ national date and time representation
%x national date representation
%X national time representation
%G year with century, starting on first Monday where week has 4 or more days.
%g year without century, starting on first Monday where week has 4 or more days.
%V week number of the year, starting on first Monday where week has 4 or more days.
Rails 4 之后推荐的另外一种做法
Rails 4 推荐在 I18N 文件中指定时间格式，比如 config/locales/en.yml
en:
time:
formats:
submitted: "%b %d %Y"
然后在 view 中使用 Rails 的 #l 辅助方法，<%= l review.created_at, format: :submitted %>。
详情可以参阅文档，http://guides.rubyonrails.org/i18n.html#adding-date-time-formats
