Among the tzdata rules for France one finds these lines: # DSH writes that a law of 1923-05-24 specified 3rd Sat in Apr at 23:00 to 1st # Sat in Oct at 24:00; and that in 1930, because of Easter, the transitions # were Apr 12 and Oct 5. Go with Shanks & Pottenger. Rule France 1922 1938 - Oct Sat>=1 23:00s 0 - Rule France 1923 only - May 26 23:00s 1:00 S Rule France 1924 only - Mar 29 23:00s 1:00 S Rule France 1925 only - Apr 4 23:00s 1:00 S Rule France 1926 only - Apr 17 23:00s 1:00 S Rule France 1927 only - Apr 9 23:00s 1:00 S Rule France 1928 only - Apr 14 23:00s 1:00 S Rule France 1929 only - Apr 20 23:00s 1:00 S Rule France 1930 only - Apr 12 23:00s 1:00 S I have done some source research, and the law of 1923-05-24 really states that. My own translation of the text is: ---- begin quote Article 1 The legal time fixed by the law of 1911 Mar 9 shall each year, following the practical details as given in articles 2,3 and 4, be advanced by sixty minutes, at 23 hours on each last Saturday of March, until the first Saturday of October at 24 hours. But in the case of an understanding with allied neighbour nations, the goverment can shift the first date to the third Saturday of April, and the second date to the third Saturday of September. --- end quote This law was not modified until 1939-09-26. But in practice it was not followed regarding the begin dates. The government issued a decret each year, where it fixed the dates for the particular year. I searched the 'Bulletin des lois de la République française' which are accessible in in digital form via the Bibliotheque National at http://gallica.bnf.fr up to 1931. As an example, I attach the décret for 1930. I think translation isnot needed. From what I have seen as sources between 1916 and 1938, so far, I can confirm that Henri Le Corre is precise and correct, regarding dates and times of change. I can also confirm the '24 hours' always means the end of the day, if a document states something like 'on date X at 24 hours'. Later versions of the décrets are explicit, like the one attached, by saying 'at twenty four hours in the night from 4th to 5th October'. I think the comment currently in the list of Rules, regarding DSH and 1930, can be removed, as Le Corre has been established as reliable for DST rules during this period up to at least 1938.