In the latest beta version, 4.00.086, we changed the behavior of the 30 day Custom Duration to mimic the eBay behavior of ending listings monthly instead of every 30 days. If you use Custom Durations in SixBit, I would recommend upgrading to this version, as currently eBay will rollover your GTC listings this month after 29 days. ie. If you started a listing on 2/2, eBay will roll it over and charge on 3/2. SixBit would only try to end the listing on 3/3, after 30 days.
We are targeting next Monday (3/9) for a full release version with this functionality.