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I think you're trying to trick us.

The input date to your question is the 25th of October, 2010 , whilst the output is the 29th of October, 2010 . Well played!

Continuing on this nit-picking thread: your times are also completely different and you're missing the seconds from the output time.

Now for the true answer.

A little factoid first though: the ISO 8601 output in Ruby is similar to the "Combined date and time" output from ISO 8601's Wikipedia page .

You've got a string and so you'll need to convert it into a Time object which you can do with to_time . Then it's simply a matter of calling iso8601 on that object to get the ISO 8601 version:

"2010-10-25 23:48:46 UTC".to_time.iso8601
The to_time method is courtesy of Rails, whilst the iso8601 is courtesy of Ruby's standard library.
                Damn! you saw through my plot to trick you folks. =P   Thanks for the great answer, i wonder why they do not just write that in the documentation!
– meow
                Oct 30, 2010 at 18:30
                @Mark Thomas: yes it was supposed to be humour. He gave an input and an output time that wouldn't match, ever. Tricksie little hobbit!
– Ryan Bigg
                Oct 30, 2010 at 20:01
                This works, but although his question asked how to do it in ruby, it didn't assume it was in the rails environment.
– courtsimas
                Feb 2, 2015 at 21:16
                Related, thanks for pointing me at 'to_time' ~ Time.now.iso8601 formats like "2010-10-25T23:48:46Z", if you want to use '+00:00' instead of 'Z' (which I had to for a legacy system) Time.now.to_time.iso8601 appears to do this.
– lucygenik
                Aug 19, 2015 at 21:40
After much experimenting, I find the Time library's parser to be better than DateTime, although the reasons escape me at the moment.  With that caveat, I always use Time rather than DateTime for this kind of stuff, and the ruby documentation is also difficult to grok as to why this is so,
require 'time'
puts Time.parse("2010-10-25 23:48:46 UTC").iso8601
"2010-10-25T23:48:46Z"
                good pure ruby solution. Just want to add, if original time is not UTC, you may want this: Time.parse("...").utc.iso8601, otherwize you'll get the time suffixed with +03:00 or some other value depending on the timezone original time string is given with. I mean instead of Z.
– akostadinov
                Jun 22, 2016 at 10:20
Note: you have to convert (parse) a time string into a time object before you can apply the to_time method.
ruby-1.9.2-p180 :016 > "2010-10-25 23:48:46 UTC".to_time.iso8601
NoMethodError: undefined method `to_time' for "2010-10-25 23:48:46 UTC":String
    from (irb):16
Correct procedure:
irb> ut = DateTime.parse("2010-10-25 23:48:46 UTC")
irb> ut.iso8601
 => "2010-10-25T23:48:46+00:00" 
Adding an answer to this super old question because if you're using Rails, there is no need to convert/parse it in the way the other answers here are telling you to do:
If it's already a timestamp (e.g., a created_at or updated_at attribute), you can directly call the iso8601 method on that timestamp (e.g., object.created_at.iso8601). 
The iso8601 method also accepts a numeric argument to display fractional digits; you would use this if you want 2020-04-06T19:16:55.604Z instead of 2020-04-06T19:16:55Z.
https://api.rubyonrails.org/v5.2.4/classes/ActiveSupport/TimeWithZone.html#method-i-iso8601
        
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