This phrase is used by people who want time to move quickly, so they may move on with their lives, ala prison release, militarydischarge, school graduation...
Sundowning is a term used to describe a decline in cognitive abilities toward the late afternoon or evening. It is a common pattern in senile dementia.
Poor Grandma tends towards sundowning around dusk; she's fairly alert in the morning but gets more confused in the evening.