firmware/src/gps/GPS.cpp

#include "GPS.h"
#include "configuration.h"
#include "time.h"
#include <assert.h>
#include <sys/time.h>

#ifdef GPS_RX_PIN
HardwareSerial _serial_gps_real(GPS_SERIAL_NUM);
HardwareSerial &GPS::_serial_gps = _serial_gps_real;
#else
// Assume NRF52
HardwareSerial &GPS::_serial_gps = Serial1;
#endif

bool timeSetFromGPS; // We try to set our time from GPS each time we wake from sleep

GPS *gps;

// stuff that really should be in in the instance instead...
static uint32_t
    timeStartMsec; // Once we have a GPS lock, this is where we hold the initial msec clock that corresponds to that time
static uint64_t zeroOffsetSecs; // GPS based time in secs since 1970 - only updated once on initial lock

void readFromRTC()
{
    struct timeval tv; /* btw settimeofday() is helpfull here too*/

    if (!gettimeofday(&tv, NULL)) {
        uint32_t now = millis();

        DEBUG_MSG("Read RTC time as %ld (cur millis %u) valid=%d\n", tv.tv_sec, now, timeSetFromGPS);
        timeStartMsec = now;
        zeroOffsetSecs = tv.tv_sec;
    }
}

/// If we haven't yet set our RTC this boot, set it from a GPS derived time
void perhapsSetRTC(const struct timeval *tv)
{
    if (!timeSetFromGPS) {
        timeSetFromGPS = true;
        DEBUG_MSG("Setting RTC %ld secs\n", tv->tv_sec);
#ifndef NO_ESP32
        settimeofday(tv, NULL);
#else
        DEBUG_MSG("ERROR TIME SETTING NOT IMPLEMENTED!\n");
#endif
        readFromRTC();
    }
}

#include <time.h>

uint32_t getTime()
{
    return ((millis() - timeStartMsec) / 1000) + zeroOffsetSecs;
}

uint32_t getValidTime()
{
    return timeSetFromGPS ? getTime() : 0;
}