%X - preferred time representation for the current locale without the date
%y - year as a decimal number without a century (range 00 to 99)
%Y - year as a decimal number including the century
%Z - time zone or name or abbreviation
%% - a literal `%' character
PROGRAMMERS NOTE: date_format is essentially a wrapper to PHP's strftime()
function. You may have more or less conversion specifiers available depending
on your system's strftime() function where PHP was compiled. Check your
system's manpage for a full list of valid specifiers.
程序员提示:date_format本质上是php的strftime()函数的一个包装。
当php被编译的时候你可以或多或少的依靠系统的strftime()转换有效的区分符。
可以查看系统手册的有效区分符的全表.
