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!"
The act of defecating through the open sunroof of a car. Originally performed by Jimmy McGill, aka Saul Goodman on "Better Call Saul" which ultimately landed him in jail for assault, destruction of property, and also possibly sex offense charges.