Hi Roser
The most common reason when you have discrepancy of "one" day is that people today (also some researchers) dont understand how "military" days often where reported...
an example: A German a/c is shot down 16:00 Day 1 and the report is able to catch the
breaking time for this day, which could be 16:00, 17:00, 18:00 (did vary).
The other side (French, British) was not able to report the incident for Day 1 because they
also did have a military breaking time and the report from this "sector" was not reported until 19:00....so it was reported Day 2...
If you read original reports you will soon catch this phenomen...
Gunnar