This phrase is used by people who want time to move quickly, so they may move on with their lives, ala prison release, military discharge, school graduation...
The inmate said "3 weeks until I flatten this time, so Hurry Sundown!"
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.