fix tests failures, which occurs short before midnight

[thet]

when running the tests short before midnight, these fails:
https://github.com/plone/plone.event/blob/master/plone/event/utils.py#L182

plone.app.event handles this issue with a patched_now function:
https://github.com/plone/plone.app.event/blob/master/plone/app/event/tests/base_setup.py#L14

[thet]

@tisto ^^

[tisto]

Thanks! :)