True that, but NeoPixels calls turn off interrupts, so millis() might not be as good as you'd think. Some libraries for smart LEDs attempt to compensate, it isn't clear how well that goes.
Don't ask how I know. 
If you want any chance of remaining in sync with the clock on the wall, the inaccuracy of the microprocessor clock cannot in any way be the basis for time keeping. add a cheap and easy enough to use RTC module, a DS3231 type is very good.
a7