You could create the NSDate from the sliders value:
NSDate *date = [[NSDate alloc] initWithTimeintervalSinceNow:(slider.value * 60)];
Then use its timeInterval to create the timer:
[NSTimer scheduledTimerWithTimeInterval:[date timeIntervalSinceNow] target:self selector:@selector(nextFunction) userInfo:nil repeats:NO];
I guess that should do it.