No, it's not an object. It is a phrase that describes time. And for durations as well as for spatial distances pure accusatives (without a preposition) are used in German: example: "Ich fahre drei Wochen in Urlaub" (English has "for three weeks" here), "Ich laufe drei Kilometer" ("I run three kilometers").
So a period of time requires the accusative? Seems odd. You aren't doing something to the day.