Common start of the first MCCH modification period

[kuehnhammer]

Context

ETSI TS 136 300 V16.2.0 (2020-07) states in 15.3.8 Synchronisation of MCCH Update Signalling via M2:

The MCE and the concerned eNBs maintain a common time reference which allows each node to be aware of the
modification period boundary within an MBSFN Area. In addition, each node maintains a counter of modification
periods which is incremented by one at each modification period boundary. This counter which is based on common
start of the first MCCH modification period, allows the MCE to indicate to the eNBs at which modification period the
MCCH update shall take place.

Problem description

It's not clear how this common start of the first MCCH modification period is determined. There is no reference to it in TS 36.443, which describes the M2 interface.

Suggested solution

Please clarify how the MCE and eNB can independently determine the (absolute) start time of the first MCCH modification period, assuming they are synchronized to a common UTC time base.