Let's say I want delay of a week it's 6.048e+8 milliseconds,how much would my delay be wrong,hope you understand my question
You should be able to delay up to just under 50 days (the number of milliseconds represented by a 32 bit number).
Some Arduinos have a ceramic resonator and the timekeeping is not very accurate (minutes or more per day). The ones with a crystal are better.
If you want to handle power failures etc., it would be better to use a Real Time Clock with battery backup, and use the alarm function or code a similar function yourself.
First off, don't use delay() at all. Use millis() to manage timing without blocking as in Several Things at a Time
Compared to a proper clock the Arduino is not a good time-keeper. You could test that by writing a program to print the value of millis() every minute and compare it with a stopwatch (most phones have stopwatches) for, say, 30 minutes or an hour.
If you want timing that is comparable with a clock you need an RTC module.
...R
So,it would be better to get a real time clock...and can you guys give me example program that counts numbers every second 1...2...3...4 and prints it to serial,because I have no idea how to write that and I would like to test arduino as a RTC to see how bad it really is 
Install the Time library. I think there is an example sketch that comes with it that you can use to test.
To see how good (or bad) the Arduino is, you could use such a sketch (untested) to count seconds to the serial monitor :
void setup() {
Serial.begin( 9600 ) ;
}
loop() {
static unsigned long secondsElapsed = 0 ;
static unsigned long countMs = 0 ;
if ( millis() - countMs > 1000UL ) {
countMs = millis() ;
Serial.print ( "Second = " ) ;
Serial.println ( secondsElapsed ) ;
secondsElapsed ++ ;
}
}
if ( millis() - countMs >= 1000UL ) {
countMs += 1000UL;
Take 0.5% accuracy as a reasonable estimate of the Uno's clock's accuracy. After a week, the clock will have drifted by 50 minutes.
Use an RTC. A DS3231 has an internal, temperature compensated crystal with a maximum accuracy of 2 ppm. That's a maximum of 1 minute of drift per year, 2,500 times better than a resonator.
Datasheet - DS3231 RTC.pdf (824 KB)
unsigned long currentMicros;
unsigned long previousMicros;
unsigned long elapsedTime;
byte hundredths;
byte tenths;
byte secondsOnes;
byte oldSecondsOnes;
byte secondsTens;
byte minutesOnes = 0;
byte minutesTens = 4;
byte hoursOnes = 1;
byte hoursTens = 1;
void setup(){
Serial.begin(115200); // make serial monitor match
currentMicros = micros();
previousMicros = currentMicros;
Serial.println ("Setup Done");
}
void loop(){
currentMicros = micros();
// how long's it been?
elapsedTime = currentMicros - previousMicros;
// Serial.print ("Elapsed: "); <------ comment out
// Serial.println (elapsedTime); <------ comment out
if ( elapsedTime >=10000UL){ // 0.01 second passed? Update the timers
elapsedTime = 0;
previousMicros = previousMicros + 10000UL;
hundredths = hundredths+1;
if (hundredths == 10){
hundredths = 0;
tenths = tenths +1;
if (tenths == 10){
tenths = 0;
secondsOnes = secondsOnes + 1;
if (secondsOnes == 10){
secondsOnes = 0;
secondsTens = secondsTens +1;
if (secondsTens == 6){
secondsTens = 0;
minutesOnes = minutesOnes + 1;
if (minutesOnes == 10){
minutesOnes = 0;
minutesTens = minutesTens +1;
if (minutesTens == 6){
minutesTens = 0;
hoursOnes = hoursOnes +1;
if (hoursOnes == 10){
hoursOnes = 0;
hoursTens = hoursTens =1;
if (hoursOnes == 4 && hoursTens ==2){
hoursOnes = 0;
hoursTens = 0;
}
}
} // minutesTens rollover check
} // minutesOnes rollover check
} // secondsTens rollover check
} // secondsOnes rollover check
} // tenths rollover check
} // hundredths rollover check
} // hundredths passing check
if (oldSecondsOnes != secondsOnes){ // show the elapsed time
oldSecondsOnes = secondsOnes;
Serial.print ("Time: ");
Serial.print (hoursTens);
Serial.print(hoursOnes);
Serial.print(":");
Serial.print(minutesTens);
Serial.print(minutesOnes);
Serial.print(":");
Serial.print(secondsTens);
Serial.println(secondsOnes);
/*
if ( hoursTens == 0 && hoursOnes == 6 ** minutesTens == 0 && minutesOnes == 0 && secondsTens == 0 && secondsOnes == 0){
// alarm time!
}
*/
} // end one second check
} // end loop
?
Sketch only produces random numbers, and locks up my IDE after some time.
Leo..
comment out this two lines of code will not produces random numbers
// how long's it been?
elapsedTime = currentMicros - previousMicros;
// Serial.print ("Elapsed: "); // <---comment out
// Serial.println (elapsedTime); // <-- comment out
if ( elapsedTime >=10000UL){ // 0.01 second passed? Update the timers
AWOL:
if ( millis() - countMs >= 1000UL ) {
Well spotted.
aarg:
countMs += 1000UL;
I'm not so sure about that ?
You all missed:
void loop() {
6v6gt:
I'm not so sure about that ?
I am. Ad nauseam.
Good:
if ( millis() - countMs >= 1000UL )
{
countMs += 1000UL;
Serial.print ( "Second = " ) ;
Serial.println ( secondsElapsed ) ;
secondsElapsed ++ ;
}
Better:
if ( millis() - countMs >= 1000UL )
{
while(millis() - countMs >= 1000UL)
{
countMs += 1000UL ;
}
Serial.print ( "Second = " ) ;
Serial.println ( secondsElapsed ) ;
secondsElapsed ++ ;
}
If the while loops more than once, secondsElapsed is ... ? 
You are always thinking.
while(millis() - countMs >= 1000UL)
{
countMs += 1000UL ;
secondsElapsed ++ ;
}
.
larryd:
You are always thinking.
Ugh. I wish that were true. I just accidentally dumped a glass of milk on my keyboard.
.
For that, good sir, you get some karma!