ESP32Time time offset table

timezone.js

/**
 * This file calculates a lookup table for all the day that daylight saving take effect.
 * The output file is a hpp file, which can use used in Arduino project.
 * If you don't have nodejs on your machine, you can use an online compiler, for example: https://www.tutorialspoint.com/execute_nodejs_online.php
 */


// number of years in the future to generate
const NB_OF_YEARS = 20;
// set the current timezone
const TZ = 'Europe/Paris';

process.env.TZ = TZ;
const now = new Date();
now.setHours(0);
now.setMinutes(0);
now.setSeconds(0);
now.setMilliseconds(1);

const endYear = now.getFullYear() + NB_OF_YEARS;
const timeOffsetTable = [];
const addNewOffsetEntry = () => {
  const offsetInt = now.getTimezoneOffset() * -1 * 60;
  console.log(`New time offset entry: ${now.toISOString()} - Offset: ${offsetInt}`);
  timeOffsetTable.push({
    time: (now.getTime() - 1) / 1000,
    offset: offsetInt,
    comment: now.toISOString(),
  });
};
addNewOffsetEntry(); // add the current entry

let lastOffset = now.getTimezoneOffset();
while (now.getFullYear() <= endYear) {
  const ONE_HOUR = 60 * 60 * 1000;
  now.setTime(now.getTime() + ONE_HOUR);
  if (now.getTimezoneOffset() !== lastOffset) {
    lastOffset = now.getTimezoneOffset();
    addNewOffsetEntry();
  }
}

const CPP_CODE = `
#define timeOffsetTableSize ${timeOffsetTable.length}
unsigned long timeOffsetTable[${timeOffsetTable.length}][2] = {
${timeOffsetTable.map(entry => `  {${entry.time}, ${entry.offset}}, // ${entry.comment}`).join('\n')}
};

// Get the current offset. Please use ESP32Time.getLocalEpoch() to get the currentEpoch
long getTimeOffset(unsigned long currentEpoch) {
  for (int i = timeOffsetTableSize - 1; i >= 0; i--) {
    unsigned long time = timeOffsetTable[i][0];
    unsigned long offset = timeOffsetTable[i][1];
    if (time <= currentEpoch) {
      return offset;
    }
  }
  return 0; // default, should never happen
}

/**
 * To use it:
 * ESP32Time rtc(0);
 * rtc.setTime(epochTime); // epochTime maybe fetched from internet using NTP
 * 
 * Then before you read the time:
 * rtc.offset = getTimeOffset(rtc.getLocalEpoch());
 * 
 * Then just read the time as normal:
 * int hour = rtc.getHour(true); // true for 24h format
 * int minute = rtc.getMinute();
 */
`;

console.log(CPP_CODE);