I want to use the following function to display MCU's uptime, but after several hours I get error.

void uptime(char *ret_clk)
  byte days = 0;
  byte hours = 0;
  byte minutes = 0;
  byte seconds = 0;

  int sec2minutes = 60;
  int sec2hours = (sec2minutes * 60);
  int sec2days = (sec2hours * 24);

  unsigned long time_delta = millis() / 1000UL;

  days = (int)(time_delta / sec2days);
  hours = (int)((time_delta - days * sec2days) / sec2hours);
  minutes = (int)((time_delta - days * sec2days - hours * sec2hours) / sec2minutes);
  seconds = (int)(time_delta - days * sec2days - hours * sec2hours - minutes * sec2minutes);

  sprintf(ret_clk, "%01dd %02d:%02d:%02d", days, hours, minutes, seconds);

after 2:55 hours I got a correct answer : 0d 02:55:27

also after 4 hours, a correct answer : 0d 04:06:44

but roughly after 12 hours i get an error: 1d 02:01:27

and after roughly 20 hours I get : 2d 23:74:26

What is wrong in uptime ?


On an 8-bit Arduino an int can only hold values in the range -32768 to +32767. One day's worth of seconds is 86400, which is far outside that range.

You should replace your sec2... variables with literals, maybe #defined - and don't forget to force the length for longer values:

#define SEC2MINUTES 60
#define SEC2HOURS 3600
#define SEC2DAYS 86400UL
  • what is the bennefit define it using #define ? whould define as usgined long save the problem ? – Guy . D May 3 at 13:04
  • 1
    @Guy.D If you put it in a variable and use that variable to calculate other variables to do your calculations with it has to calculate them every single iteration. By using #define (or you could use const unsigned long if you wish) it calculates it at compile time (which I shortcut by just calculating it myself). – Majenko May 3 at 13:06
  • what might be that cause getting this result :upTime[0d 10:123:33] ? code was corrected as you suggested – Guy . D May 5 at 16:13

The millis function increments rapidly, hence it easily exceeds the maximum value of int - -32768 to +32767.

So, use unsigned long data types for millis and other stuff that's large.

4 billion and some change is the max value that unsigned long data types can store, hence no overflowing till about 49 days.

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