FWIW - I was working an issue the other day where I was inserting a date time stamp into a mongoDB, it looked like this before inserting: "2012-02-15 15:00:00 +0100". When I pulled it out it looked like this:
"2012-02-15 15:00:00 UTC". I was then passing it to Time.parse(), and I was getting a similar error "....undefined method `gsub' for nil:NilClass...". I ended up stripping the UTC off, then parsing it and it worked fine. Not likely to be at all related, some of the details are similar enough that perhaps it will help you debug some more. Good hunting....