Same problem here. The bug is quite obvious: the backup file name is
generated *after* the warning is printed.
Here's an excerpt of xserver-xorg.postinst around line 1685:
warn "$XORGCONFIG has been customized, but we need to make updates. \
Backing up your config to $BACKUP_XORGCONFIG. If we screw something up, \
restore using this file."
BACKUP_XORGCONFIG="$XORGCONFIG.$(date '+%Y%m%d%H%M%S')"
cp "$XORGCONFIG" "$BACKUP_XORGCONFIG"
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Ciao, Flavio