Hrm. I entered the DW 24 hours after last but am now unable to go to the Dream Isle (says I have to wait a day from when I last entered). I've waited a good 20 minutes, so I'm now more than 24 hours from when I went to the isle yesterday.
So does this mean they allow DW access 23 hours after the time runs out but only allow Dream Isle access 24 hours after the time runs out? I'm sure I've managed to maintain an ~24 hour cycle before now, so perhaps it's just bugged.