Starting from version 2.9, you can choose whether files that are overwritten or removed from the destination should be moved to the trash or a custom folder.
A custom backup folder must be on the same device as the destination. If the backup folder is inside the destination (or the source for two-way syncs), it will be automatically excluded.
When moved to the backup folder, the current timestamp is appended to each filename. The timestamp has the format yyyy-MM-dd[T]HH-mm-ss+ZZZZ, which is independent of the current system locale and takes into account the timezone as well. This timestamp will be used in order to determine the oldest backup files that are to be removed when the destination drive has no more space available.