在某些情况下，shell 日期 "-n hours"与 "n hours ago"不同

Question

在 shell 中，当我打印这个时

date -d "2016-11-23 13:05 -1 hours "  "+%Y-%m-%d %H:00:00"

我得到 2016-11-23 23:00:00.奇怪的!

当我打印这个

date -d "2016-11-23 13:05 1 hours ago"  "+%Y-%m-%d %H:00:00"

我得到 2016-11-23 12:00:00 .

为什么它们不同？我认为他们都是2016-11-23 12:00:00 .

Answer 1

这是因为负数被视为时区的偏移量，而不是 13:05 .在我的时区 MET(格林威治标准时间以东一小时)，这是我得到的:

$ date -d "2016-11-23 13:05 -1 hours "  "+%Y-%m-%d %H:00:00"
2016-11-23 16:00:00
$ TZ=GMT date -d "2016-11-23 13:05 -1 hours "  "+%Y-%m-%d %H:00:00"
2016-11-23 15:00:00
$ TZ=GMT-1 date -d "2016-11-23 13:05 -1 hours "  "+%Y-%m-%d %H:00:00"
2016-11-23 16:00:00
$ TZ=GMT-1 date -d "2016-11-23 13:05 -2 hours "  "+%Y-%m-%d %H:00:00"
2016-11-23 17:00:00

时区偏移量通常指定为四位数字，如

Sun, 29  Feb 2004  16:21:42 -0800

但显然 date(1) 也对 -1 感到满意。

从手册页:

DATE STRING

The --date=STRING is a mostly free format human readable date string such as "Sun, 29 Feb 2004 16:21:42 -0800" or "2004-02-29 16:21:42" or even "next Thursday". A date string may contain items indicating calendar date, time of day, time zone, day of week, relative time, relative date, and numbers. An empty string indicates the beginning of the day. The date string format is more complex than is easily documented here but is fully described in the info documentation.