The problem seems to be be that the data back in old history is shifted even more, so somewhere between there was a change.
How long is a trade week? Always 5*24h (in forex) or did it change over the year(s)? If so it would be possible to find the shift (by looking at first and last bar of a week)
Use the CME forex futures trading hours for reference.
The CFD from FXCM and other brokers align to the CME hours
Forex hours use DST.
