I have ported the code to ATTiny85.  To make a long story short, the first version 

hat could fit the Digispark used 99% of flash 

The second version calling functions that dealt with PROGMEM and using tiny versions for RTC, neopixel and I2C (wire) libraris still took me 95% of the flash.

I have then replaced the Wire and RTC libraries with my own code and now the sketch takes 87% of the flash. 

There's still some room for improvements, but that's good enough! for the moment.

Sources available at project's Github repository