This is a very short post pointing to a good plug-in for validating time stuff in Rails. One-line summary: use validates_timeliness.
I wanted to validate the format of a datetime field in one of my models, so I tried validates_date_time and saw that Rails doesn't have it installed. I googled validates_date_time, and stumbled upon the plug-in with the same name by Jonathan Viney.
I installed the plugin, fixed the unit tests to run on Rails 2.2 with sqlite3, and then proceeded to write my validation, complete with unit tests. All the code is included, for your pleasure. After some booring debugging, I came to the conclusion the plugin can't handle invalid formats on new models, because of how it's using _before_type_cast to determine whether its parser failed or succeeded.
I googled for datetime validation in rails 2.2, hoping that someone fixed the bug. I didn't find a fix, but instead stumbled over the validates_timeliness plugin, written by Adam Meehan. In case you're seeing this in plain text, the link to the plug-in is http://github.com/adzap/validates_timeliness/
I hope Google indexes my post, and that it helps you avoid going through the same trouble that I have.
The code I used for this post: