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This isn't wrong - /any/ time zone has a constant 24 hour delta before those two points in time.

What is changing is your time zone from, say, GMT to BST.

It provides the right output if the right timezone is given:

I see 24 hours (not the correct 23) when I follow that link.

Edit: Now I see that the link has both times in EST, which is not on point. See chrisrhoden's link in a sibling to this comment.

A move between non-DST and DST is considered a time zone change - in my case from EST to EDT.

This time zone change is listed there. It says 1:00pm EST to 1:00pm EDT on the page linked, and so it is incorrect.