Just did this online after flying back on 15 August. First two attempts using 15 August as the entry date were rejected, so tried my previous date (three years ago and the one on previous reports) - also rejected. Then remembered that we had arrived just before midnight on the 15th, so did a fourth report using 16 August as the entry date and this was accepted - even though my passport is stamped as the 15th. Not previously had any issue filing online since the new system was introduced. Therefore, if doing online, start early to give yourself time for a few attempts.