TZ has this information in file backzone


# From Paul Eggert (2014-10-11):


# See Asia/Ho_Chi_Minh for the source for most of this
        data.  Also, guess


# (1) Cambodia reverted to UT +07 on 1945-09-02, when
        Vietnam did, and


# (2) they also reverted to +07 on 1953-11-09, the date
        of independence.


# These guesses are probably wrong but they're better
        than guessing no


# transitions there.


Zone    Asia/Phnom_Penh 6:59:40 -       LMT     1906 Jul 
        1


                        7:06:30 -       PLMT    1911 May 
        1


                        7:00    -       +07     1942 Dec
        31 23:00


                        8:00    -       +08     1945 Mar
        14 23:00


                        9:00    -       +09     1945 Sep 
        2


                        7:00    -       +07     1947 Apr 
        1


                        8:00    -       +08     1953 Nov 
        9


                        7:00    -       +07
A good friend of mine born in 1951 in Phnom Pen is the
        adoptive daughter of late Cambodian ruler Prince Sihanouk.


Being very interested in astrology, this friend of course
        much engaged in knowing the correct time zone for her birth.
The information above would set it at 8h east of GMT for 1947
        - 1953.
My friend writes (and I have bolded the essential phrase):


According to my father xxxx, during the French
          protectorate, Cambodia’s legal time was that of Hanoi. [..]

        
My [adoptive] father Sihanouk confirmed the above
          in 2011 when I
          drafted my essay on King Sihanouk. I needed to ask him many
          questions, including details
          regarding our time zone, especially 5-6 August 1951. [...]

    
King Sihanouk said that, during the Japanese
          occupation,
          starting with what historians refer to as “le coup de force du
          9 mars 1945”,
          Cambodia, like the entire French Indochina, used Tokyo time
          zone. After Japan
          surrendered, 2 September 1945, Cambodia fell under French rule
          again and
          adopted Hanoi time zone again.

    
However, on 7 January 1946, Sihanouk and
            Tioulong managed
            to obtain a status of “internal autonomy” from the
            government of Charles de Gaulle.
            Although many fields remained under the administration of
            the French (customs,
            taxes, justice, defence, foreign affairs, etc.), the
            Cambodian administration
            was responsible for religious matters and traditional
            celebrations, which
            included our calendar and time. The time zone was GMT + 7
            and no DST was
            applied.

    
After Sihanouk and Tioulong achieved full
          independence,
          on 9 November 1953, GMT + 7 was maintained.
-- end quote

        
In view of this, I would tend to
            eliminate the line

                                    8:00    -       +08     1953 Nov  9

            Of course, it is here like in many other cases the question
            what TZ wants to represent: offical time maintained by the
            colonial administration, or 'time on the ground' used by the
            local population for their own records, like a royal birth
            event.

            
            I do not expect action by Paul Eggert, as this is only
            backzone data.Still. I consider it worthwile to be recorded
            in the TZ mailing ist history.