RTC Zero losing time

EDIT: This is very weird - I have swapped for another (virtually identical board) and the below issue does not present (i.e. the clock keeps counting). Is the board's RTC defective?

Hi

I have an Arduino Zero and I am using this library to keep a track of the time using the inbuilt 'rtc'. However, I have noticed that my rtc stops counting time when I call the sendText() function. The code below demonstrates the problem:

  #include <Arduino.h>

#include <RtcAlarmScheduler.h>
#include <RadioFunctions.h>

// clock object
RTCZero remoteStationClock;

// alarms and ISRs
#define CS_M0  8
#define RST_M0 4
#define INT_M0 3

// Radio settings
const int FREQ = 434;
const int TX_PWR = 20;
const int SPREAD_FACTOR = 12;
const int BANDWIDTH = 125E3; 
const int CODING_DENOM = 5;
const bool CRC_ON = true;

RH_RF95 remoteStationRadio(CS_M0, INT_M0);

char TEST_TXT[] = "HELLO TEST WORLD BLAH BLAH BLAH. GOODBYE!HELLO TEST WORLD BLAH BLAH BLAH. GOODBYE!HELLO TEST WORLD BLAH BLAH BLAH. GOODBYE!";

bool sendText(const char* txt, RH_RF95& radio)
{
   if ( !radio.send( (uint8_t*)txt, strlen(txt)) )
   {
     return false;
   }

  if ( !radio.waitPacketSent() )
   {
     return false;
   }
  radio.setModeRx(); // put back into RX mode
  return true;
}

void setup() {
    
    Serial.begin(9600);
    const int C_TIME = 1622494907;

    Serial.print("TIME IS: ");
    Serial.println(C_TIME);

    remoteStationClock.begin();
    remoteStationClock.setEpoch(C_TIME);

    setupRadio(remoteStationRadio,RST_M0, FREQ, TX_PWR, SPREAD_FACTOR, BANDWIDTH, CODING_DENOM, CRC_ON);
    Serial.println("RADIO SETUP");

}

void loop() {
   
    delay(10000);

    Serial.print("EPOCH START:");
    Serial.println(remoteStationClock.getEpoch());
    uint32_t startSW = millis();
    
     if ( !sendText(TEST_TXT, remoteStationRadio) )
     {
       Serial.println("FAILED TO SEND");
     }

    Serial.print("SEND TIME (MSECS):");
    Serial.println(millis() - startSW);
    Serial.print("EPOCH END:");
    Serial.println(remoteStationClock.getEpoch());
    Serial.println("");

}

This produces the following output:

EPOCH START:1622494917
SEND TIME (MSECS):4926
EPOCH END:1622494917 // why isn't this 1622494917 + 4.9 secs?

EPOCH START:1622494927
SEND TIME (MSECS):4926
EPOCH END:1622494928

EPOCH START:1622494938
SEND TIME (MSECS):4925
EPOCH END:1622494938

Does anyone know what might be going on here? Why does the clock stop during this function?

Thanks

Arduino M0 and other SAMD boards with M0 bootloader loose RTC time at reset because the bootloader resets RTC.

Not a great RTC, then.
Good information.

The board isn't being reset - at setup the time is set from an external source. During execution, it seems to lose time.