I need a program to count days, from day 1 to day 21 for my project.
I'm making an artificial incubator and I need to show the days on an LCD.
You can do that easily using Real Time Clock IC. ds1307/ds3231 ic will do the job. Adafruit has Arduino library for that. easy to interface.
I don't supprt using millis() function. you will lose your time if the system power downs or mcu resets. Using RTC chip is safe option as it has backup battery for time keeping.