Oracle bones are the ancient records that first attest the sexagenary cycle for recording days in China.
The earliest evidence comes from divination inscriptions on Shang-dynasty oracle bones, dating to about 1100 BC. These records use the cycle’s stem-branch combinations to identify days, making the system one of the oldest continuously documented methods of calendrical notation in China.
The cycle combines ten Heavenly Stems with twelve Earthly Branches. Their coordinated sequence produces 60 terms, and the day count continued through later periods. Nearly every oracle-bone inscription includes a date in this format, while use for day records remained common through the Zhou dynasty and into the Han period.
A frequent mix-up is with silk manuscripts from Mawangdui. Those are important evidence for the cycle’s use in recording years: documents from a tomb sealed in 168 BC contain a sexagenary grid marking historical events. They are therefore later evidence for a different application. Oracle bones are the answer when the question asks about the earliest attestation for recording days.