The British introduced Double British Summertime (GMT plus 2 Hours) in May 1941 and DBST was used for the rest of the war during each summer.
It was also used by the USAAF in NW Europe (and US Army & USN)
DBST was used between the following dates:
4th May 1941until 10th August 1941.
5th April 1942 until 9th August 1942
4th April 1943 until 15th August 1943
2nd April 1944 until 17th September 1944
2nd April 1945 until 15th July 1945
Dates when German Summer Time was equal to Double British Summer Time.
4th May 1941 until 10th August 1941.
5th April 1942 until 9th August 1942
2nd November 1942 until 31st December 1942.
1st January 1943 until 29th March 1943
4th April 1943 until 15th August 1943
4th October 1943until 31st December 1943.
1st January 1944 until 2nd April 1944
4th April until 17th September 1944
2nd October 1944 until 31st December 1944.
1st January 1945 until 2nd April 1945.