my personal understanding.
one does not need to stay a full 24 hours for a day to be counted. if at the second 23:59:59 sept 11, you are physically inside canada, sept 11 should be counted, otherwise an ABSENCE.
e.g.
1 -- leaving Canada sept 11 at 8AM, coming back same day 11PM, sept 11 is counted.
2 -- leaving Canada sept 11 at 8AM, coming back sept 12 at 11PM, sept 11 is not counted, but sept 12 is counted
3 -- entering Canada sept 11 at 8AM, leaving on same day at 11PM, not counted.
The residence calculator should provide the most accurate results.