Subtracting 1 month from October 31st results in September 31st. Since the date doesn't exist, it is normalized: September 31st is "one day after September 30th", which results in October 1st.
Thanks for explanation. To me it makes more sense that going one month back from 31 oct, or last day of oct, would give last day of sep instead of first day of oct. But now that I'm aware of this feature I can work around it if needed.
Ilja
