First, welcome to the Forum.

Second, if you have the box, there should be a four-digit number on label on the end of the box.
The number indicates the date of manufacture - the first number is the year and the next three numbers are the day.
It is assumed that you know the decade of manufacture.

My M&P reads 3017 - the 017th day of 2013 - January 17, 2013.

It's the four-digit number underneath the six-digit SKU number.