XMLSchema and ISO-8601: Not a Match Made in Heaven

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Ever used the ISO-8601 standard to format dates in, for example, your API? And ever thought you'd be "safe" for all client applications, because you adhere to an ISO standard?

Then I got a little surprise for you.

<xs:element name="date_added" type="xs:dateTime" />

XMLSchema doesn't implement ISO-8601 completely, something I only found out after 3 hours of debugging. When using the dateTime type, the XMLSchema definition says the following which is somewhat vague:

This representation simplifies interval arithmetic and leap-year calculation for dates before
the common era (which may be why astronomers and others interested in such calculations with
the proleptic Gregorian calendar have adopted it), and is consistent with the current edition
of [ISO 8601].

What they (should) mean to say is that their accepted input format is one of the formats ISO-8601 describes. The trick is in the, optional, timezone part. ISO-8601 allows for the timezone part to be just four digits with a plus sign in front: +0900. This means that a date/time combination could be formatted as follows:


This format will, however, not validate through XMLSchema. It expects the most verbose syntax that is allowed by the standard, which includes a colon : between the second and third digit of the timezone. This makes kind of sense, because in most other parts of the standard format, the separator : or - can be omitted as well.

The fun part, however, is yet to come. When working in PHP, which I do a lot, there's a problem. In PHP, there's the \DateTime class to work with, you've guessed it, dates and times.

$dateAndTime = new \DateTime('now');

This class has a format() method that takes a textual format pattern to determine the format you want. For your convenience, some preconfigured formats have been defined. One example, which I used to use a lot, was the ISO-8601 one.

const string ISO8601 = "Y-m-d\TH:i:sO" ;

The documentation says the following about it:

ISO-8601 (example: 2005-08-15T15:52:01+0000)

Fair enough. This adheres to the standard. But when using it to generate an XML output that will be checked using XMLSchema, it will just fail.

The guys at PHP noticed this problem and solved it for us. There's another predefined format, which is called W3C. It simply adds a : and you're good to go and use XMLSchema!

$dateAndTime = new \DateTime('now');
echo $dateAndTime->format(\DateTime::W3C);

// Will output 2014-02-06T07:06:00+00:00

From now on, I use the W3C one everywhere. It adheres to the ISO-8601 standard just as well, but obviously allows for more client application to run without problems!