How do I get Ruby to parse time as if it were in a different time zone?

You could just append the UTC timezone name to the string before parsing it:

require 'time'
s = "11/23/10 23:29:57"
Time.parse(s) # => Tue Nov 23 23:29:57 -0800 2010
s += " UTC"
Time.parse(s) # => Tue Nov 23 23:29:57 UTC 2010