date将秒数转换成日期的字符串表达形式

这里要用到date函数的第三种形式，下面是获得当前日期

(set 'today (date (date-value) 0 "%Y%m%d"))

(date-value) 返回的是1970年0点累计的秒数，作为date函数的第一个参数

第二个参数是偏移的分钟，0表示没有偏移，就是当前时区的现在时间。 这个参数很有用，可以计算昨天的时间，也可以计算其他时区的时间

第三个参数定义日期格式，参考下表：

formatdescription%aabbreviated weekday name according to the current locale%Afull weekday name according to the current locale%babbreviated month name according to the current locale%Bfull month name according to the current locale%cpreferred date and time representation for the current locale%dday of the month as a decimal number (range 01–31)%Hhour as a decimal number using a 24-hour clock (range 00–23)%Ihour as a decimal number using a 12-hour clock (range 01–12)%jday of the year as a decimal number (range 001–366)%mmonth as a decimal number (range 01–12)%Mminute as a decimal number%peither 'am' or 'pm' according to the given time value or the corresponding strings for the current locale%Ssecond as a decimal number 0–61 (60 and 61 to account for occasional leap seconds)%Uweek number of the current year as a decimal number, starting with the first Sunday as the first day of the first week%wday of the week as a decimal, Sunday being 0%Wweek number of the current year as a decimal number, starting with the first Monday as the first day of the first week%xpreferred date representation for the current locale without the time%Xpreferred time representation for the current locale without the date%yyear as a decimal number without a century (range 00–99)%Yyear as a decimal number including the century%ztime zone or name or abbreviation (same as %Z on Win32, different on Unix)%Ztime zone or name or abbreviation (same as %z on Win32, different on Unix)%%a literal '%' character

下面是使用了偏移参数计算昨天的日期：

(set 'yesterday (date (date-value) (- (* 24 60)) "%Y%m%d"))

24小时乘以60分钟，结果加上负号