> Can you explain why math.floor rather than builtin round is the correct function to use?

To avoid breaking existing scripts that use `.strftime('%s')` on Linux, OSX,
see msg221385:

  >>> from datetime import datetime, timezone
  >>> dt = datetime(1969, 1, 1, 0,0,0, 600000, tzinfo=timezone.utc)
  >>> '%d' % dt.timestamp()
  '-31535999'
  >>> round(dt.timestamp())
  -31535999
  >>> dt.astimezone().strftime('%s') # <-- existing behavior
  '-31536000'
  >>> '%d' % math.floor(dt.timestamp())
  '-31536000'
  >>> import calendar
  >>> calendar.timegm(dt.astimezone(timezone.utc).timetuple())
  -31536000