I understand what the sentence The house is a full day's journey from here. means but I'm wondering what day's is a shortcut of in it. It doesn't match any pattern I know.

A couple of examples:

• He's = he is
• Let's = let us
• Mary's car. = the car belongs to Mary
• Day's = it sounds like something belongs to a day to me and this is what I don't understand. Shouldn't it be The house is a full day of journey from here. instead?