From the abstract of the spec: This document defines a date and time format for use in Internet protocols that is a profile of the ISO 8601 standard for

From the abstract of the spec: This document defines a date and time format for use in Internet protocols that is a profile of the ISO 8601 standard for representation of dates and times using the Gregorian calendar [emphasize is mine] https://tools.ietf.org/html/rfc3339 https://tools.ietf.org/html/rfc3339