user68988: Here I have made a standalone ATmega328p clock without
any RTC module using the breadboard ATmega328p , LCD I2C , and some buttons which I would like to turn on for atleast an year (supposed to be without any reset or power-interrupts).
I am powering my ATmega using an old mobile charger (5v , 500mA) of a old broken Android.
However I am not using any millis() or something alike these in the clock code,
So how long will my clock run if is it supposed to be online 24/7
Should I reset it sometimes?
If yes , when, after how many days should I reset it?
And do I need a coolant system for it? (I guess not)
Thanks for your precious time .

user4143: 
So how long will my clock run if is it supposed to be online 24/7

Until you turn it off or a stray neutrino corrupts the flash memory, or the flash memory degrades (20-100 years or so depending on temperature).

Should I reset it sometimes? If yes , when, after how many days should I reset it?

No.

And do I need a coolant system for it? (I guess not)

No. Not unless it's in a sealed container with no ventilation and a hot environment.
However
Your RTC-less "clock" will probably not be particularly accurate, unless you have some external source of time (NTP, GPS, etc) as the Arduino's internal timing is not very accurate, and built on a breadboard it's even less so due to excess stray capacitance.

68988: <p>Here I have made a standalone ATmega328p clock without
any RTC module using the breadboard ATmega328p , LCD I2C , and some buttons which I would like to turn on for atleast an year (supposed to be without any reset or power-interrupts).</p>
<p>I am powering my ATmega using an old mobile charger (5v , 500mA) of a old broken Android.</p>
<p>However I am not using any millis() or something alike these in the clock code,</p>
<p>So how long will my clock run if is it supposed to be online 24/7</p>
<p>Should I reset it sometimes?
If yes , when, after how many days should I reset it?</p>
<p>And do I need a coolant system for it? (I guess not)</p>
<p>Thanks for your precious time .</p>
 4143: <blockquote>
<p>So how long will my clock run if is it supposed to be online 24/7</p>
</blockquote>
<p>Until you turn it off or a stray neutrino corrupts the flash memory, or the flash memory degrades (20-100 years or so depending on temperature).</p>
<blockquote>
<p>Should I reset it sometimes? If yes , when, after how many days should I reset it?</p>
</blockquote>
<p>No.</p>
<blockquote>
<p>And do I need a coolant system for it? (I guess not)</p>
</blockquote>
<p>No. Not unless it's in a sealed container with no ventilation and a hot environment.</p>
<p><strong>However</strong></p>
<p>Your RTC-less &quot;clock&quot; will probably not be particularly accurate, unless you have some external source of time (NTP, GPS, etc) as the Arduino's internal timing is not very accurate, and built on a breadboard it's even less so due to excess stray capacitance.</p>
