Using millis() like in Arduino.

Here are two simple functions to use when you want to use timestamps for time measurement.

uint32_t millis(void)
  return(app_timer_cnt_get() / 32.768);

and for comperison of current with given timestamp with rollover correction (thanks to Nicolas Brunner for the correction):

#define OVERFLOW ((uint32_t)(0xFFFFFFFF/32.768))

uint32_t compareMillis(uint32_t previousMillis, uint32_t currentMillis)
  if(currentMillis < previousMillis) return(currentMillis + OVERFLOW + 1 - previousMillis);
  return(currentMillis - previousMillis);

You can use it like:

uint32_t myTimeStamp = millis();
//... some code here
if(compareMillis(myTimeStamp, millis()) > 2000)
  printf("2 seconds have passed\n");

Here are unit tests