Exiv2

ISO 8601 includes colons.
http://www.pbcore.org/PBCore/dateCreated.html, section GUIDELINES FOR USAGE, shows asmuch.
This is how the guide (http://www.exiv2.org/metadata.html) has it written under "IPTC datasets defined in Exiv2":

Designates in the form CCYYMMDD the earliest date the provider intends the object to be used. Follows ISO 8601 standard.

This is how ReleaseDate,ExpirationDate, and DateCreated are all described. Use this syntax verbatim or as shown and you will get this error:
*Warning: Unsupported date format*
*Warning: Iptc.Application2.DateCreated: Failed to read Date value "20090301"*
I'm sure it's as true for 0.20 as it was for previous versions that the format shown in the Guide will throw an error.

The correct format for a set command for any of these IPTC tags is as follows (at least they worked for me):

set Iptc.Application2.DateCreated Date 2009-03-01
set Iptc.Application2.TimeCreated Time 06:28:16
set Iptc.Application2.ReleaseDate Date 2010-09-01

It's very nice to not have to use a GUI image editor to add or modify date and time created in the IPTC data sphere. It'd be even nicer if the guide pages were more accurate as to error-free syntax of dates and times.

Steve W