"come nighttime" is correct and usable in written English.
You can use it to refer to the time when night begins, usually when the sun sets or when darkness starts to fall. For example, "Come nighttime, the city lights up and the streets become alive with people and activity."
Come nighttime, you're on your own.
Come nighttime, the wind's really starting to blow.
Come nighttime, all is well with the world again.
Where to Party Colmados are what Dominicans call the local grocery stores, which come nighttime, double as neighborhood hangouts.
Good teachers are like jugglers keeping a dozen balls in the air so come nighttime, with alarm set for 6am to finish grading papers, memories of the day that's gone – including the students who didn't understand something, forgot their lunch or were embarrassed by wrong answers – become sleep-resistant barriers.
Come nighttime, I explore more modern precincts.
A first-time visitor to Joplin today might gasp at the sight of some of its neighborhoods: the destroyed and vacant houses; the stripped trees; the front steps leading to emptiness; the crushed cars evoking the dead; the many streets that, come eerie nighttime, have no power, no light, no life.
All observations in the hospital took place during daytime hours, while observation at CritiCall Ontario took place overnight, since the majority of urgent calls come at nighttime hours.
"That guy came at nighttime to ask my grandpa for money so that he could take me to my parents, who are in Denver," Leonel said.
Ken Stringfellow: For my money, I come back to Nighttime.
For his part, Terrence Doyle made Death Valley come alive through nighttime critters ("coyotes, chuckwallas, roadrunners") and "thickets of oleander and salt cedar".
