Real-time clock (RTC) is a crucial function in automation control systems, enabling features such as automatic watering and scheduling. ESP8266-based controllers typically lack an RTC module, which would increase costs and affect the product's affordability. To address this issue, Zhang Zhuyue designed a software-based RTC clock module that utilizes the ESP8266's mktime and localtime functions to convert between calendar and timestamp. This innovative solution enables automatic cloud calibration, ensuring accurate timekeeping without the need for an external RTC chip.

Source: https://dev.to/zhangzhuyue/rtc-design-with-automatic-cloud-calibration-for-simple-programmable-controller-based-on-esp8266-2lo9