Re: How do I compare two NSDates using NSPredicate Core Data
A date-time is a date-time, regardless of how it is
displayed or entered or obtained from the system.
The whole purpose of the NSDate object is to allow easy
comparison, and determination of intervals by subtraction...
well, a couple of the main purposes... and be able to
display dates and intervals as appropriate to the context.
Calendar systems and date display formats don't lend
themselves to such calculations, and change-overs from
one calendar to another have made it more difficult.
On Fri, 2010/04/09, Ashley Clark acl...@ghoti.org wrote:
From: Ashley Clark acl...@ghoti.org
Subject: Re: How do I compare two NSDates using NSPredicate Core Data
To: Sean McBride s...@rogue-research.com
Cc: Cocoa-dev@lists.apple.com
Date: Friday, 2010 April 9, 12:52
On 2010 Apr 5, at 19:02, Sean McBride wrote:
On Sun, 2010 Apr 04 23:15:16 -0400, Michael A. Crawford said:
Thus far I've gotten away with using
-predicateWithFormat and scalar
values. I now need to compare a couple of
NSDate instances but am not
sure how to code it up with NSPredicate.
Consider me a 'visual' learner.
I'm pretty sure you can use just , , ==, etc.
with NSDates in NSPredicates.
One thing that many people overlook is that NSDates
*always* include a time component. And if that time
wasn't specified explicitly it typically defaults
to a preset value set in the editing control (if
one was used) or the current time at the time the
date string was parsed.
So, if you were expecting to compare a set of NSDate
objects to be on a particular date irrespective of
their time value... you have a bit of work to do.
A simple way to approximate the test is to look at
a time range from midnight to midnight-1 second
of the following day.
That of course leaves another issue aside though,
that of differing timezones. All NSDate objects are
converted to the local timezone by default and the
original timezone information is not stored in the
object, so an NSDate created as {2010/01/01 23:59 EDT}
when read back at some later point when the computer
is now on the west coast (PDT) would show as
{2009/12/31 20:59 PDT}.
All of that to say, comparing dates with predicates
(or at all) is hard.
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Re: How do I compare two NSDates using NSPredicate Core Data
On Apr 12, 2010, at 9:35 AM, Jeffrey Oleander wrote: A date-time is a date-time, regardless of how it is displayed or entered or obtained from the system. The whole purpose of the NSDate object is to allow easy comparison, and determination of intervals by subtraction... Yup. An NSDate is really just a fancy wrapper around a floating-point number (a CFAbsoluteTime) representing the number of seconds since 1/1/2001. (OK, an NSDate also remembers a time zone, but that's just for converting to human-readable form, it doesn't affect the actual time stored.) —Jens___ Cocoa-dev mailing list (Cocoa-dev@lists.apple.com) Please do not post admin requests or moderator comments to the list. Contact the moderators at cocoa-dev-admins(at)lists.apple.com Help/Unsubscribe/Update your Subscription: http://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to arch...@mail-archive.com
Re: How do I compare two NSDates using NSPredicate Core Data
On Apr 5, 2010, at 7:02 PM, Sean McBride wrote:
On Sun, 4 Apr 2010 23:15:16 -0400, Michael A. Crawford said:
Thus far I've gotten away with using -predicateWithFormat and scalar
values. I now need to compare a couple of NSDate instances but am not
sure how to code it up with NSPredicate. Consider me a 'visual' learner.
I'm pretty sure you can use just , , ==, etc. with NSDates in NSPredicates.
One thing that many people overlook is that NSDates *always* include a time
component. And if that time wasn't specified explicitly it typically defaults
to a preset value set in the editing control (if one was used) or the current
time at the time the date string was parsed.
So, if you were expecting to compare a set of NSDate objects to be on a
particular date irrespective of their time value... you have a bit of work to
do. A simple way to approximate the test is to look at a time range from
midnight to midnight-1 second of the following day.
That of course leaves another issue aside though, that of differing timezones.
All NSDate objects are converted to the local timezone by default and the
original timezone information is not stored in the object, so an NSDate created
as {2010/01/01 23:59 EDT} when read back at some later point when the computer
is now on the west coast (PDT) would show as {2009/12/31 20:59 PDT}.
All of that to say, comparing dates with predicates (or at all) is hard.
Ashley
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Re: How do I compare two NSDates using NSPredicate Core Data
On Fri, 9 Apr 2010 12:52:21 -0500, Ashley Clark said:
Thus far I've gotten away with using -predicateWithFormat and scalar
values. I now need to compare a couple of NSDate instances but am not
sure how to code it up with NSPredicate. Consider me a 'visual' learner.
I'm pretty sure you can use just , , ==, etc. with NSDates in
NSPredicates.
One thing that many people overlook is that NSDates *always* include a
time component. And if that time wasn't specified explicitly it
typically defaults to a preset value set in the editing control (if one
was used) or the current time at the time the date string was parsed.
So, if you were expecting to compare a set of NSDate objects to be on a
particular date irrespective of their time value... you have a bit of
work to do. A simple way to approximate the test is to look at a time
range from midnight to midnight-1 second of the following day.
That of course leaves another issue aside though, that of differing
timezones. All NSDate objects are converted to the local timezone by
default and the original timezone information is not stored in the
object, so an NSDate created as {2010/01/01 23:59 EDT} when read back at
some later point when the computer is now on the west coast (PDT) would
show as {2009/12/31 20:59 PDT}.
All of that to say, comparing dates with predicates (or at all) is hard.
I get what you're saying, but a date represents an exact moment in time,
irrespective of location or time zone or any of that. Sometimes people
may use an NSDate when they probably should be using NSDateComponents,
which you can easily do in a Core Data context using a 'transformable'
attribute. Comparing NSDateComponents is harder because you also need a
calendar, etc.
--
Sean McBride, B. Eng s...@rogue-research.com
Rogue Researchwww.rogue-research.com
Mac Software Developer Montréal, Québec, Canada
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Re: How do I compare two NSDates using NSPredicate Core Data
On Sun, 4 Apr 2010 23:15:16 -0400, Michael A. Crawford said: Thus far I've gotten away with using -predicateWithFormat and scalar values. I now need to compare a couple of NSDate instances but am not sure how to code it up with NSPredicate. Consider me a 'visual' learner. I'm pretty sure you can use just , , ==, etc. with NSDates in NSPredicates. -- Sean McBride, B. Eng s...@rogue-research.com Rogue Researchwww.rogue-research.com Mac Software Developer Montréal, Québec, Canada ___ Cocoa-dev mailing list (Cocoa-dev@lists.apple.com) Please do not post admin requests or moderator comments to the list. Contact the moderators at cocoa-dev-admins(at)lists.apple.com Help/Unsubscribe/Update your Subscription: http://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to arch...@mail-archive.com
