First you eat a lot of cereal and save the boxtops. Then you send them off, together with $1.70 postage and handling, and wait eight to ten weeks. Then your secret decoder ring comes in the mail. Some assembly required, batteries not included. Once your parents put it together, you put it on and shift it into "date code" mode, then apply it directly to your forehead, and then those date codes read out as calendar dates! Simple.
Actually, these date codes are days on a 10,000-day calendar wherein day (0000)1 is 29 July 1961. They count up from there. I have an Excel spreadsheet that lets you put in the date code and it spits out the calendar date. There's a web version here. Note you have to do a reality check on the result, because sometimes a 4-digit date code is simply a regular day and month (such as "1024" meaning 10/24, that is 24 October.