date and time method

require 'date'

require 'time'

Time.now  or Time.new

-->  Fri May 02 16:50:08 +0800 2008

Time.now.strftime('%Y-%m-%d')

-->  "2008-05-02"

SECOND_IN_A_DAY

兩個時間相減得到的單位是秒 

Date.today:

%Y, %y:  year

%b, %B: month

%m : month(number)

%d, %e:  day of month

%a, %A: dat name( such as Tue)

%H, %I: hour

%M: minute

%S: second

%c:  equals "%a %b %d %H:%M:%S %Y"

%x:  equals "%m/%d/%y"