The easiest way to know the release date of the phone is to look at the documents to him. Find a warranty card or receipt. There will be mandatory indicate the number when you made this purchase.
If the documents are missing, do it the other way: dial the number *#0000#. On the screen appears the message. The first field is the phone model, the second version of this release, but then the date you need. But it is worth considering that if you did the firmware on your phone, after dialing the number will appear in the date when you made the update last time.
If you have a phone of Sony Ericsson, the release date to find easy. Open the back panel cover and remove the battery. On its surface, find the line which reads the number format 00W00 (e.g. 06W21 - 2006 21 weeks).
If you have a mobile brand Nokia, then to find the production date, proceed as follows: turn off the phone and remove the sim card. Then put it back in the nest. After restarting the phone, booted, will prompt you to enter the current date and time. You will see the date the default is release date your mobile.
If you have a smartphone, there is need to recruit a different team than the specified in the first step. In this case, you write *#06# and the screen will display the imei of your mobile, data which will contain the date of issue.
If you got the sim card from the phone and rebooted it after this, then please note that when enabled, the program will show you the date, which means when your phone went on sale. Most likely this number will coincide with the release date (i.e. the year the same), but keep in mind that a month out of mobile may be different.