To check iPhone manufacturing date you just need to know the serial number of iPhone. Lets see how to check iphone serial number first and then how the serial numbers tells you the manufacturing date of iPhone. Using this guide you can find manufacture date of iPhone 2G, 3G and 3GS.
How to check iPhone Manufacture date: (iPhone 2G, iPhone 3G, iPhone 3GS)
Tap on “Settings” icon > General > About.
Here you will find serial number of you iPhone as show below in screenshots.
You just need 3rd, 4th and 5th digits of iPhone serial number to know the manufacture date of iPhone. So in the iPhone screenshot the 3rd digit is ‘8’ shows manufacturing year ‘2008’ and the 4th-5th digits ’46’ shows the manufacturing week, 46th week. So the iPhone manufacture date is 46th week of year 2008.
Another example: If the serial number is xx736xxxxxx then manufacture year is 2007 and 36th week.
Update: Apple change the serial number scheme for CDMA iPhone 4 and iPhone 4S.