基本使用格式：
date [-d "time-to-display"] +"format-to-display"
-d指定要显示的时间，如果不指定默认为当前时间；后边的+后的字符串为指定的输出格式，类式C的print的格式。常用示例如下：
date -d "1 day ago" +"%Y-%m-%d"
date -d "30 day ago" +"%Y-%m-%d"
date -d "+ 1 day" +"%Y-%m-%d"
date -d "- 1 month" +"%Y-%m-%d"
date -d "" +"%Y-%m-%d"
date -d "" +"输出为：%Y-%m-%d"
支持的输出项：
%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 :: )
%C century; like %Y, except omit last two digits (e.g., )
%d day of month (e.g, )
%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 (..)
%I hour (..)
%j day of year (..)
%k hour ( ..)
%l hour ( ..)
%m month (..)
%M minute (..)
%n a newline
%N nanoseconds (..)
%p locale’s equivalent of either AM or PM; blank if not known
%P like %p, but lower case
%r locale’s -hour clock time (e.g., :: PM)
%R -hour hour and minute; same as %H:%M
%s seconds since -- :: UTC
%S second (..)
%t a tab
%T time; same as %H:%M:%S
%u day of week (..); is Monday
%U week number of year, with Sunday as first day of week (..)
%V ISO week number, with Monday as first day of week (..)
%w day of week (..); is Sunday
%W week number of year, with Monday as first day of week (..)
%x locale’s date representation (e.g., //)
%X locale’s time representation (e.g., ::)
%y last two digits of year (..)
%Y year
%z +hhmm numeric timezone (e.g., -)
%:z +hh:mm numeric timezone (e.g., -:)
%::z +hh:mm:ss numeric time zone (e.g., -::)
%:::z numeric time zone with : to necessary precision (e.g., -, +:)
%Z alphabetic time zone abbreviation (e.g., EDT)