Occasional Hyphen Dash in millis

Using Arduino Programming Questions
1

Writing a program to read X,Y,Z from an Adafruit 10 DOF IMU, and write it to a file on an SD card. I want to track flight time in millis as well. My string is basically
millis,X,Y,Z,pressure,altitude

My loop:

void loop() {
  
  // time counter
  time = millis();
  
  // Get IMU data
  getIMUInfo();

  String logString = String(time);
  logString += getIMUInfo();
  Serial.println(logString);

}

The millis portion of the output contains hyphens in the "tens" position, on a random basis:

21-0.08,-0.59,10.40,844.77,1507.88
940.00,-0.55,10.36,844.77,1507.88
1660.00,-0.59,10.43,844.80,1507.59
239-0.04,-0.55,10.40,844.82,1507.40
3130.04,-0.51,10.43,844.78,1507.78
3860.00,-0.51,10.43,844.81,1507.49
4580.00,-0.51,10.43,844.87,1506.92
531-0.04,-0.43,10.43,844.85,1507.11
6050.00,-0.55,10.51,844.79,1507.69
6770.00,-0.51,10.40,844.82,1507.40
7500.00,-0.55,10.43,844.77,1507.88
8240.00,-0.51,10.43,844.77,1507.88
8970.00,-0.51,10.43,844.80,1507.59
9690.04,-0.55,10.47,844.81,1507.49
10420.00,-0.55,10.36,844.77,1507.88
11160.04,-0.59,10.36,844.74,1508.17
11880.04,-0.51,10.40,844.81,1507.49
12610.04,-0.55,10.51,844.76,1507.97
13350.04,-0.55,10.40,844.80,1507.59

Any idea what might be going on? Any way to work around it, other than dropping the reading?

2

Post all the code.

The number being printed is negative, suggesting integer overflow.

3

that means --> declare the variable time holding millis() as an unsigned long

4

You put no comma between the millis and the rest of the IMU string. You are seeing the minus sign of the next number adjacent.

5

OK that is embarrassing. Correct answer - I forgot the comma between millis() and the other entries. Thank you for catching that, I should have reviewed my code more carefully. 6 am on a holiday weekend is apparently NOT a good time for debugging.

6

MorganS:
You put no comma between the millis and the rest of the IMU string. You are seeing the minus sign of the next number adjacent.

Nice catch.