Linux2009.05.12 14:10
7.6 Relative item in date strings

Relative items adjust a date (or the current date if none) forward or backward. The effects of relative items accumulate. Here are some examples:

1 year
1 year ago
3 years
2 days

The unit of time displacement may be selected by the string `year' or `month' for moving by whole years or months. These are fuzzy units, as years and months are not all of equal duration. More precise units are `fortnight' which is worth 14 days, `week' worth 7 days, `day' worth 24 hours, `hour' worth 60 minutes, `minute' or `min' worth 60 seconds, and `second' or `sec' worth one second. An `s' suffix on these units is accepted and ignored.

The unit of time may be preceded by a multiplier, given as an optionally signed number. Unsigned numbers are taken as positively signed. No number at all implies 1 for a multiplier. Following a relative item by the string `ago' is equivalent to preceding the unit by a multiplier with value -1.

The string `tomorrow' is worth one day in the future (equivalent to `day'), the string `yesterday' is worth one day in the past (equivalent to `day ago').

The strings `now' or `today' are relative items corresponding to zero-valued time displacement, these strings come from the fact a zero-valued time displacement represents the current time when not otherwise change by previous items. They may be used to stress other items, like in `12:00 today'. The string `this' also has the meaning of a zero-valued time displacement, but is preferred in date strings like `this thursday'.

[링크 :  http://sunsite.ualberta.ca/Documentation/Gnu/tar-1.13/html_chapter/tar_7.html#SEC111]

date --date=now
date --date=today # same thing
date --date='3 seconds'
date --date='3 seconds ago'
date --date='4 hours'
date --date='tomorrow'
date --date='1 day'
date --date='1 days'
date --date='yesterday'
date --date='1 day ago'
date --date='1 days ago'
date --date='1 week'
date --date='1 fortnight'
date --date='1 month'
date --date='1 year'

[링크 : http://www.linuxforums.org/forum/linux-programming-scripting/111764-yesterdays-date-linux.html]
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