Created originally on Bitbucket by barrywhart
Changesets for this Pull Request have not been imported, because it had been already declined on Bitbucket. Marked as closed by the import user.
This PR addresses https://bitbucket.org/pypy/pypy/issues/2634/datetime-example-where-pypy3-is-30x-slower for the 2.7 version of Pypy. Because datetime didn't implement in-place add, each use of += was creating a new, temporary datetime object.