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Yes, it did change, though I'm not sure why.

From a mail to roundup-devel a while ago:
--
Johannes: The date interval code seems to broken. When
running the demo, the activity column is always in the
future ('in a moment', 'in three hours'). The tests run fine
though.

Richard: That'll be more the fault of the pretty() function.
I suspect that it mis-interprets the sign of the interval
after Anthony's changes which reversed the sign result from
date operations.
--

Assuming there was a good reason for reversing the sign,
your inversion of the subtraction seems like the correct fix
to me.