Refined adjustment of millis() / MS_COUNTER clock on AVR to be closer to correct for a wider range of pixel counts.  It's now correct roughly within a handful of seconds per hour.  The exact accuracy depends on the number of pixels, what else your code is doing and how long it takes doing it, and of course, the ambient temperature.  You STILL can't make an alarm clock without adding an external RTC, but you can probably do a respectable syncronization of an animation to a 5-minute pop song.  Total cost of this change is 1 byte of SRAM and about 40 bytes of program space.  If you want to disable clock correction to save space, #define NO_CORRECTION 1 before including FastLED.h
Refined adjustment of millis() / MS_COUNTER clock on AVR to be closer to correct for a wider range of pixel counts.  It's now correct roughly within a handful of seconds per hour.  The exact accuracy depends on the number of pixels, what else your code is doing and how long it takes doing it, and of course, the ambient temperature.  You STILL can't make an alarm clock without adding an external RTC, but you can probably do a respectable syncronization of an animation to a 5-minute pop song.  Total cost of this change is 1 byte of SRAM and about 40 bytes of program space.  If you want to disable clock correction to save space, #define NO_CORRECTION 1 before including FastLED.h

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