From the ISO-8601:2004(E) Specification:
4.2.2.4 Representations with decimal fraction
If necessary for a particular application a decimal fraction of hour, minute or second may be included. If a decimal fraction is included, lower order time elements (if any) shall be omitted and the decimal fraction shall be divided from the integer part by the decimal sign specified in ISO 31-0, i.e. the comma [,] or full stop [.]. Of these, the comma is the preferred sign.
Simple enough. So according to this spec, fractions of a second are preferred to be written using a comma separating the whole and decimal parts, such as 2014-01-01T00:00:00,123. However it seems that just about everywhere, only a decimal point (aka "full stop") is accepted!
Now I'm sure there are some languages or libraries that took this into account, and I know in many cases you can supply the full details of the format yourself. But it seems like such a glaring oversight of the specification and it appears that a wide variety of programmers have made the same mistake. Is there a reason why this is the case, other than pure human error?
Below is a list of where I tested. Feel free to edit the question to augment my list if you find any others. Thanks.
.NET / C#
DateTime dt = DateTime.Parse("2014-01-01T00:00:00,123");
Throws a FormatException with the message "String was not recognized as a valid DateTime". The same thing with a period instead of a comma parses successfully.
JavaScript Date Object
Tested in latest (as of this writing) Chrome, Internet Explorer, Firefox and Node.js:
var dt = new Date('2014-01-01T00:00:00,123');
Returns "Invalid Date". Using a period instead works fine.
JavaScript with moment.js
var valid = moment("2014-01-01T00:00:00,123").isValid();
Returns false. Using a period instead returns true.
PHP
echo strtotime('2014-01-01T00:00:00,123');
Returns an empty string. Using a period instead works fine.
Ruby
require 'time'
puts Time.iso8601("2014-01-01T00:00:00,123")
Gives a runtime error. While Time doesn't keep fractional seconds, it shouldn't error - and indeed if a period is used instead, it works.