ISO 11104:2011

ISO 11104:2011 establishes a small number of standardized recommended time code formats for use in data interchange applications between agencies of the CCSDS. ISO 11104:2011 does not address timing performance issues such as stability, precision, accuracy, etc.
Time codes are digital representations of time information. ISO 11104:2011 describes four standard recommended time codes (one "unsegmented" and three "segmented" codes) which use the international standard second as the fundamental unit of time. An unsegmented time code is a pure binary count of time units and fractional time units from a starting time called the "epoch". A segmented time code is one in which the count of time units and fractional time units is accumulated in two or more cascaded counters which count modulo of various bases and start from the epoch.