I'm using a Nodemcu Esp8266 v1.0 to measure some data and sending it with WiFi. I'm programming it with the Arduino library.
In order to save battery, I use Deep Sleep. Some of the measurements use interruptions to wake-up the device BEFORE the time implemented in the function:
ESP.deepSleep(20e6); // 30e6 is 30 seconds
So, to keep a consistent frequency of updating the data, I would need to know the time between Deep Sleeps. Do you know any function or memory direction to know how much time RTc counted? I could use EEPROM, but I would need where can I measure the time.