How we decode
Manufacturers hide the date of manufacture inside the serial number, and each one does it differently. We read the date with written rules, one set per manufacturer, and we show which characters gave the answer.
Every rule has a source
Each rule comes from published references: manufacturer documents first, then utility and government guides, then established home inspection references. The sources for every format are listed below and on each brand page.
Every rule is tested
Each format is checked against real serial numbers with known dates, and against impossible ones, such as a month 13. The site will not build if a single test fails.
When sources disagree
We use only what the good sources agree on, and we say where they differ. For example, older A. O. Smith month letters for September to December are described inconsistently, so we flag those months as less certain.
When a serial could mean more than one date
Some manufacturers reuse their year codes. A Bradford White year letter comes around every 20 years. When that happens we show every possible date, most likely first where there is a reason to prefer one, and explain how to tell them apart. We never pick one quietly.
When we don't know
If a serial does not fit any format we know, we say so. If we recognize a format but have not confirmed how it stores the date, we say that too, rather than guess.
The brand comes from you
Different brands use the same shape of serial number to mean different things. A Lennox and a Carrier serial can look identical. So we always decode with the brand you choose, never by guessing from the shape.