[Courtois Laurent]

Thanks Andrey for your answer.

I will try to formalize your answer.

Before 1930 Moscow time is GMT + 2 (universal time).
By a decree of the Council of People's Commissars of June 16th, 1930, the clock hands having been advanced by one hour, it is the "Derektnoe" times.
Moscow is then in GMT + 3.
Thus the offset is +1 in summer and +2 in winter.

Thank you very much for allowing me to understand.