[1] ebml-specification/specification.markdown at master · Matroska-Org/ebml-specification () https://github.com/Matroska-Org/ebml-specification/blob/master/specification.markdown#date-element

A Date Element MUST declare a length of either zero octets or eight octets. If the EBML Element is not defined to have a default value, then a Date Element with a zero-octet length represents a timestamp of 2001-01-01T00:00:00.000000000 UTC [@!RFC3339].

The Date Element stores an integer in the same format as the Signed Integer Element that expresses a point in time referenced in nanoseconds from the precise beginning of the third millennium of the Gregorian Calendar in Coordinated Universal Time (also known as 2001-01-01T00:00:00.000000000 UTC). This provides a possible expression of time from 1708-09-11T00:12:44.854775808 UTC to 2293-04-11T11:47:16.854775807 UTC.

[2] ebml-specification/specification.markdown at master · ietf-wg-cellar/ebml-specification · GitHub, https://github.com/ietf-wg-cellar/ebml-specification/blob/master/specification.markdown#date-element

[3] Date Element specs: fix miscalculation & clarification about leap seconds · Issue #411 · ietf-wg-cellar/ebml-specification, https://github.com/ietf-wg-cellar/ebml-specification/issues/411

[4] >>3 閏秒の規定がなかったため、閏秒のないUTCと明確化されました。