Were you already logged into the
DCL site prior to the actual midnight (eastern) time? If so, the DCL system still recognizes the date as the day you logged in.
Also, it's no unusual for the check in window, or the activity booking window to be delayed in opening. Sometimes it's one, or the other, occasionally, both will be delayed. No rhyme or reason, just happens.
Only solution - just keep trying. Or wait until morning.