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bug#26101: Counterproductive calculation order in date
From: |
Ulf Zibis |
Subject: |
bug#26101: Counterproductive calculation order in date |
Date: |
Wed, 15 Mar 2017 23:37:59 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.7.0 |
Am 15.03.2017 um 15:40 schrieb Assaf Gordon:
To give more details about the inter working of date adjustments:
Much thanks for outlining these details in great clarity.
You'd like to be able to do date calculations in some predictable way,
Yes!
Currently the result is not predictable because the order of processing
the terms is not defined transparently.
in your case to be able to change a file's timestamp.
Not any more as thanks to Eric I now know how to do that by only help of
touch.
However,
consider that adjusting by any scale (years/months/days/hours/minutes) will
have side-effects.
Adjusting by months can shift days (e.g. "2016-03-30 - 1 month" is still march).
Adjusting by days can shift hours (e.g. on Day light saving time),
Adjusting by hours can stay in the same day (e.g. "+24 hours" on a when day light saving
results in 25 "hours").
etc.
It is the edge-cases that make day adjustment tricky (e.g. "+1 month" is not "+30 days", "+1
day" is not always "+24 hours", etc.).
If you can come up with reliable and predictable rules for date adjustment that
will not have these unexpected results - there's definitely room for
improvements.
I do not see any problems with adjustment when determining the
processing order from left to right, but I'm not a real specialist to
that subject.
However, there's also the existing behavior and backwards compatibility that
will need to be taken into account.
This may be an important issue.
On the other hand, such side effects could also result from arbitrary
code changes, as the processing order to follow is nowhere determined.
Maybe an intelligent search over all shell scripts of a typical Linux
installation could discover the usage of such cases.
-Ulf
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