user26574: This is a very basic question but I cannot find answer for this. I have a couple of Arduino Nano boards lying with me. With keen scrutinizing I found that a few chips on board where numbered as Atmega 328P-AU 1645 and some were numbered Atmega 328P-AU 1644. Can somebody please tell me the difference the number after AU makes ? I know the difference between an AU and PU chip. 

user7508: Looks to me like a date code: YYWW, where YY are the last two digits of the year and WW is the ISO week number.

26574: <p>This is a very basic question but I cannot find answer for this. I have a couple of Arduino Nano boards lying with me. With keen scrutinizing I found that a few chips on board where numbered as Atmega 328P-AU 1645 and some were numbered Atmega 328P-AU 1644. Can somebody please tell me the difference the number after AU makes ? I know the difference between an AU and PU chip. </p>
 7508: <p>Looks to me like a date code: YYWW, where YY are the last two digits of the year and WW is the ISO week number.</p>
