There are a few areas where we use the rule מקצת היום ככולו - "part of the day is considered as if it were the entire day"
I have some questions regarding how this rule works:
Is there a general rule, situation or scenario as to when this rule can be applied? For example, we don't say, "observing part of Yom Kippur is as if you observed all of it. Therefore, you can eat after noon time."
The only place, offhand, that I can think of where this rule is applied is regarding the observance of shiva. On the last (7th) day, mourners don't complete the entire 7th day, but are done in the morning. My question is - if the day has already begun the night before (at sundown), why can't the last day of shiva end an hour (or whatever period) after nightfall?