Solved // Datum von gestern ohne Library berechnen

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So geht es mit Unix Time. Aus der Time Library geklaut und etwas angepasst:

#define LEAP_YEAR(Y) ( ((1970+Y)>0) && !((1970+Y)%4) && ( ((1970+Y)%100) || !((1970+Y)%400) ) )
#define SECS_PER_MIN  (60UL)
#define SECS_PER_HOUR (3600UL)
#define SECS_PER_DAY  (SECS_PER_HOUR * 24UL)

const uint8_t monthDays[] = {31,28,31,30,31,30,31,31,30,31,30,31};

struct tm_t
{ 
	uint8_t Second; 
	uint8_t Minute; 
	uint8_t Hour; 
	uint8_t Wday;
	uint8_t Day;
	uint8_t Month; 
	uint16_t Year;
};

void unixToDate(uint32_t unixtime, struct tm_t& tm)
{
	uint8_t year, month, monthLength;
	uint32_t days;

	tm.Second = unixtime % 60;
	unixtime /= 60;
	tm.Minute =  unixtime % 60;
	unixtime /= 60;
	tm.Hour = unixtime % 24;
	unixtime /= 24;
	tm.Wday = ((unixtime + 4) % 7) + 1;

	year = 0;  
	days = 0;
	while((unsigned)(days += (LEAP_YEAR(year) ? 366 : 365)) <= unixtime) {
		year++;
	}
	tm.Year = year;

	days -= LEAP_YEAR(year) ? 366 : 365;
	unixtime -= days;

	days=0;
	month=0;
	monthLength=0;
	for (month=0; month<12; month++) {
		if (month==1) {
			if (LEAP_YEAR(year)) {
				monthLength=29;
			} else {
				monthLength=28;
			}
		} else {
			monthLength = monthDays[month];
		}

		if (unixtime >= monthLength) {
			unixtime -= monthLength;
		} else {
			break;
		}
	}
	tm.Year += 1970;
	tm.Month = month + 1;
	tm.Day = unixtime + 1;
} 

uint32_t dateToUnix(struct tm_t& tm)
{   
	unsigned int i;
	uint32_t seconds;

	tm.Year -= 1970;
	seconds = tm.Year*(SECS_PER_DAY * 365);
	for (i = 0; i < tm.Year; i++) {
		if (LEAP_YEAR(i)) {
			seconds +=  SECS_PER_DAY;
		}
	}

	for (i = 1; i < tm.Month; i++) {
		if ( (i == 2) && LEAP_YEAR(tm.Year)) { 
			seconds += SECS_PER_DAY * 29;
		} else {
			seconds += SECS_PER_DAY * monthDays[i-1];
		}
	}
	seconds+= (tm.Day-1) * SECS_PER_DAY;
	seconds+= tm.Hour * SECS_PER_HOUR;
	seconds+= tm.Minute * SECS_PER_MIN;
	seconds+= tm.Second;
	return seconds; 
}

void printDate(struct tm_t& tm)
{
	Serial.print(tm.Day); Serial.print('/'); Serial.print(tm.Month); Serial.print('/'); Serial.println(tm.Year);
	Serial.print(tm.Hour); Serial.print(':'); Serial.print(tm.Minute); Serial.print(':'); Serial.println(tm.Second);
}

void setup()
{
	Serial.begin(9600);
}

void loop()
{
	tm_t tm;
	tm.Day = 27;
	tm.Month = 6;
	tm.Year = 1971;
	tm.Hour = 12;
	tm.Minute = 31;
	tm.Second = 25;

	printDate(tm);
	unsigned long time = dateToUnix(tm);
	Serial.println(time);
	Serial.println();

	time = time - 7 * SECS_PER_DAY;
	Serial.println(time);

	unixToDate(time, tm);
	printDate(tm);
	Serial.println();

	delay(5000);
}

Es gibt auch einfachere Algorithmen, aber nicht alles was man im Netz (vor allem in Foren) findet geht auch :slight_smile: