Thanks all you folks!
The missing piece of information, as guessed by Paul Eggert:
TZ=America/Sao_Paulo (Brazil/East).
All commands from the report were executed a few hours before reporting,so it
was Oct,15... a few days from entering Daylight Saving time.
Tried all your suggestions they
This problem was reproduced in several versions of 'date', and the oldest one I
could reproduce the problem was 6.10.
But it doesn´t happen in older ones (tried on 'date' 5.97)
Examples:
# ./date +%Y --date=20101018
2010
# ./date +%Y --date=20101016
2010
# ./date +%Y --date=20101017
./date:
