For iPhone, even if the model is the same, the quality may be different. Those who are worried about buying a bad-quality iPhone are recommended to check it out!
Open Settings, then tap General, find About, and drop down to find the serial number.
The fourth and fifth digits of the serial number represent the production date.
The fourth digit is the number of years of production.
K = the first half of 2013, L = the second half of 2013.
M = the first half of 2014, N = the second half of 2014.
P = the first half of 2015, Q = the second half of 2015.
R = the first half of 2016, S = the second half of 2016.
T = the first half of 2017, V = the second half of 2017.
W = the first half of 2018, X = the second half of 2018.
The fifth place is the number of weeks of production.
1= week 1, 2= week 2, 3= week 3, 4= week 4,
5= week 5, 6= week 6, 7= week 7, 8= week 8,
9= week9 , C= week 10, D= week 11, F= week 12,
G= week 13, H= week 14, J= week 15, K= week 16,
L= week 17, M= week 18, N= week 19, P= week 20,
Q= week 21, R= week 22, T= week 23, V= week 24,
W= week 25, X= week 26, Y= week 27.
Remember to check this method for the iPhone you just bought, so that you can know the origin of the phone and calculate whether the phone is a new phone or the warranty period of the phone.
James Chow 
Hazel Yu 
T4TeCH 