Examples of GNU Date

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本文主要介绍了date命令的多种使用示例，包括打印不同日期、设置系统时钟、按特定格式输出日期等。还涉及日期与秒数的转换，可用于数据排序、绘图和比较，同时说明了时间区信息对日期解释的影响。

原文： http://www.labri.fr/Perso/~strandh/Teaching/USI/Common/Sh-utils/sh-utils_65.html
作者：David MacKenzie et al.

Examples of `date`

Here are a few examples. Also see the documentation for the `-d' option in the previous section.

To print the date of the day before yesterday:
```
date --date='2 days ago'
```
To print the date of the day three months and one day hence:
```
date --date='3 months 1 day'
```
To print the day of year of Christmas in the current year:
```
date --date='25 Dec' +%j
```
To print the current full month name and the day of the month:
```
date '+%B %d'
```
But this may not be what you want because for the first nine days of the month, the `%d' expands to a zero-padded two-digit field, for example `date -d 1may '+%B %d'' will print `May 01'.
To print a date without the leading zero for one-digit days of the month, you can use the (GNU extension) - modifier to suppress the padding altogether.
```
date -d=1may '+%B %-d'
```
To print the current date and time in the format required by many non-GNU versions of date when setting the system clock:
```
date +%m%d%H%M%Y.%S
```
To set the system clock forward by two minutes:
```
date --set='+2 minutes'
```
To print the date in the format specified by RFC-822, use `date --rfc'. I just did and saw this:
```
Mon, 25 Mar 1996 23:34:17 -0600
```
To convert a date string to the number of seconds since the epoch (which is 1970-01-01 00:00:00 UTC), use the `--date' option with the `%s' format. That can be useful in sorting and/or graphing and/or comparing data by date. The following command outputs the number of the seconds since the epoch for the time one second later than the epoch, but in time zone five hours later (Cambridge, Massachusetts), thus a total of five hours and one second after the epoch:
```
date --date='1970-01-01 00:00:01 UTC +5 hours' +%s
18001
```
Suppose you had not specified time zone information in the example above. Then, date would have used your computer's idea of the time zone when interpreting the string. Here's what you would get if you were in Greenwich, England:
```
# local time zone used
date --date='1970-01-01 00:00:01' +%s
1
```
If you're sorting or graphing dated data, your raw date values may be represented as seconds since the epoch. But few people can look at the date `946684800' and casually note "Oh, that's the first second of the year 2000."
```
date --date='2000-01-01 UTC' +%s
946684800
```
To convert such an unwieldy number of seconds back to a more readable form, use a command like this:
```
date -d '1970-01-01 946684800 sec' +"%Y-%m-%d %T %z"
2000-01-01 00:00:00 +0000
```