How the decoder works, and how it's checked

A wrong date on a named brand is the worst thing this site could do, so nothing goes live on trust. Every rule comes from a source we link, and every rule is tested against real serial numbers whose dates we know from somewhere else.

Where the rules come from

Manufacturers build their brands on shared production lines, so a few dozen date codes cover more than a hundred brand names. For each code we start with the manufacturer's own pages, tools, manuals and recall notices, then check the eras they leave out against inspector references. Each brand page links the sources its rules came from. The rules are facts; the wording, examples and drawings on this site are our own.

How a rule is checked before it goes live

1. We collect real serial numbers whose manufacture date is known independently of our rule: the date printed on the label next to the serial, the manufacturer's own statement or example, or two independent sources that agree. A reference site decoding a serial doesn't count, because that's someone applying a rule, not evidence for it.
2. Someone other than the person who collected each serial re-reads its source and confirms the serial and the date.
3. The decoder runs against every one of them on every build. A rule goes live only when at least 3 checked serials confirm it and none fails, and a brand family only with at least 10. A serial that once failed stays in the test set for good.
4. A brand gets its own page only with its own confirmed example serials. A brand without them is still decoded, under its family.

What the decoder does when it isn't sure

• Codes that repeat. Some makers reuse a year letter every 20 years, or write only the last digit of the year. The decoder shows every date the serial could mean and asks for a clue (the install year, or something printed on the label) instead of picking one.
• Formats it doesn't know. A serial that matches no checked rule gets "format not recognized" and the manufacturer's own lookup where there is one. Never a guess.
• Typos. It ignores spaces, dashes and "S/N", and reads a letter O as zero or I as one only when the serial doesn't match as typed.
• Weeks. Codes that give a week are read from January 1 (week 1 is January 1 to 7); a week that spans two months is described as late in one or early in the next.
• Made, not installed. The date is when the unit was built. Most go in within a year, but not all.

Rule sets live today

42 checked serial numbers in all. Every build re-runs them.

Corrections

If a serial didn't decode or the date looks wrong, send it to us. We review every submission; when one reveals a format or an era we didn't have, it becomes a new rule and a new test. We re-read every manufacturer source at least once a year and after any acquisition.

Lifespans

The lifespan band on each result comes from published life-expectancy references, linked on each lifespan guide. It's a typical range, not a prediction for your unit.