Another website bug.
The online rebooking tool has a dateline bug. For example, if you change a flight that is on Apr 24, it will say "Apr 26". And you can go all the way through and it will apply your flight choices, but also move your travel 2 days later..
I've seen this many times, and corrected it each time. Today I forgot to fix the date and messed up a booking... and I'm on the phone to get it fixed.