You can thank Steve Coates for the following, which I've found incredibly useful over the years:
The Germans used CET (Central European Time, which they called MEZ = »mitteleuropäische Zeit«) and GST (German Summer Time)
CET = GMT + 1 hour
GST = GMT + 2 hours
German time varied as follows:–
To
1/4/40 - CET
From
1/4/40 - GST
2/11/42 - CET
29/3/43 - GST
4/10/43 - CET
3/4/44 - GST
2/10/44 - CET
2/4/45 - GST
18/11/45- CET