On closer inspection, I used Ashhar's suggestion and looked at pskreporter.info and compared the transmission records of a station callig CQ and my sBitx FT8 response to his CQ. The transmissions were offset by 15 seconds, not 30 seconds. So it should be working correctly. I probably just need more patience or more power to get a response because my call is from southern California, which is not really high on most operators' lists for a QSO. California hams are about as rare as Japanese ones.
It would be helpful though if the listed times on the web interface transmission roster showed the presumed start time for received transmissions as well as the start times of home transmissions.
Jack, N6LN
John Terrell, N6LN