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I had to deal with handling of various date formats for S3Hub and totally munged it the first couple passes. This post lists some things to be aware of when parsing and formatting HTTP and XML style dates correctly using the Cocoa API’s.

HTTP Date

Some quick background on HTTP dates first. S3 requires sending a ‘Date’ header field as part of an authenticated request. (This and signing the request is a common countermeasure against a replay attack.) The spec says, the format of the date should be “one of the RFC 2616 formats (http://www.ietf.org/rfc/rfc2616.txt)”.

HTTP applications have historically allowed three different formats
for the representation of date/time stamps:

Sun, 06 Nov 1994 08:49:37 GMT ; RFC 822, updated by RFC 1123
Sunday, 06-Nov-94 08:49:37 GMT ; RFC 850, obsoleted by RFC 1036
Sun Nov 6 08:49:37 1994 ; ANSI C’s asctime() format

The first format is preferred as an Internet standard and represents
a fixed-length subset of that defined by RFC 1123 [8] (an update to
RFC 822 [9]). The second format is in common use, but is based on the
obsolete RFC 850 [12] date format and lacks a four-digit year.
HTTP/1.1 clients and servers that parse the date value MUST accept
all three formats (for compatibility with HTTP/1.0), though they MUST
only generate the RFC 1123 format for representing HTTP-date values
in header fields. See section 19.3 for further information.

So these are the standard formats. When sending a formatted date you should only use RFC1123 but should be able to handle any of the formats listed. This is just a way to maintain compatibility while trying to move everything to the latest format. If you are wondering what the difference is, RFC 1123 updated 822 by changing the time zone from hour format (±0000) to GMT format (GMT).

For example, the ‘Date’ or ‘Expires’ header fields for requests have this (RFC1123) format. Replies might have an HTTP date format in a Last-Modified or Date header as well.

ISO Date

XML data which has formatted date fields might use a date format which is defined by the ISO8601 standard and looks like ‘2006-02-03T16:45:09.000Z’.

Date parsing in Cocoa

Prior to 10.4, the Cocoa API supported “strftime-style conversion specifiers”. In 10.4, they added support for the Unicode Technical Standard #35 (version tr35-6).

The pre-10.4 strftime patterns look like: “%m/%d/%y” and the Unicode standard looks like “MM/dd/yyyy”. The default behavior (for backwards compatibility) is strftime format. In order to use the Unicode standard (which you will, its way better), you need to set the formatter behavior to NSDateFormatterBehavior10_4 either by calling setFormatterBehavior or setting the classes formatter behavior globally. Do NOT use the initWithDateFormat:allowNaturalLanguage: constructor, because it will give you a pre-10.4 date formatter. Using the strftime style of pattern when configured for the 10.4 (Unicode) behavior will fail silently.

So fast forward a few weeks, an S3Hub user told me he was getting authentication errors (invalid signing) trying to connect. I couldn’t replicate the problem but I eventually got the raw HTTP request header it was sending and the Date header looked like.

Fr, 06 Jun 2008 08:49:37 GMT.

Turns out the user is in Germany and when I change my locale to something German, I can reproduce the problem. Date formatters themselves can have specific styles (and symbols) that are modified by the locale. In the German locale the weekday symbol for friday is ‘Fr’ instead of English locale which is ‘Fri’. So when you use a date formatter in this way (with day or month symbols, for example) make sure to set the locale to en_US. The final date formatter looks like:

Update: See GHNSDate+Parsing gh-kit category on github for a collection of these date formatters.

I’ve been looking for an Amazon S3 client for the Mac for awhile, that had a couple key features that seem to be missing from apps like Cyberduck or Transmit. Mostly I wanted a better way to manage permissions and make it easier to share files with my friends, and I needed something to throttle my bandwidth so I can leave beefier transfers running without nuking my Gears of War latency.

You can download it over at: http://s3hub.com

And, also, its an excuse to do some Cocoa again now that Objective-C 2.0 is out. Garbage collection, declarative properties, and fast enumeration help remove a lot of cruft and let you focus more on making things work. Also there are alot of new and really nice API’s in Leopard like NSOperationQueue.

A major feature I don’t have yet is sync’ing (syncing is hard); but I think that should come in the next version hopefully. I have some ideas about generating rss feeds or some way to track new files which I think could be really useful. If you have any ideas or find any bugs, leave a post on the group.

I ported the time_ago_in_words helper for to a category (mixin) for NSString. Its nothing profound but I couldn’t find it anywhere.

Update: Moved to github and is part of GHKit. You can find this helper: GHNSString+TimeInterval.m

If you are getting the:

Test host '/path/to/App exited abnormally with code 127 (it may have crashed).'

error in XCode 3.1 (beta) when you try to run your unit tests then you need to:

1. Get Info for the Unit Tests target.
2. Choose the Build tab.
3. Set the Architectures value to Native Architecture of Build Machine.