How do I move (not convert) a time into a new time zone?

I have a DateTime d and a string tz that is a valid ActiveSupport::TimeZone key (e.g. "UTC", "Eastern Time (US & Canada)", "America/Chicago", etc.). How do I get a new DateTime d2 such that everything is the same, but the offset of d2 now reflects that of the time zone corresponding to tz?

Note that I do not want to convert the time into the new time zone. I want a new DateTime with everything else equal, but with the offset different.

For example, if I have d = <today @ 5:00 PM UTC>, a开发者_开发百科nd tz = 'Central Time (US & Canada)', what I want is d2 = <today @ 5:00 PM -0600>, not d2 = <today @ 11:00 AM -0600>.

Given tz, you can convert that to an offset as a fraction of a day:

offset = ActiveSupport::TimeZone.new(tz).utc_offset / 60 / 60 / 24.0
=> -0.25 # for 'Central Time (US & Canada)'

You can then pass this into DateTime.civil along with existing details from d to create the new object:

d2 = DateTime.civil(d.year, d.month, d.day, d.hour, d.min, d.sec, offset)