My ship date has been brought up 8 days without them telling me. Does anyone have an explanation?

You signed on the dotted line, the minute you did that you belong to the military. They will send you when, where and how to get the best results THEY need to accomplish. Might have had some failures for a certain class and grabbed you as a filler. Who knows, get use to it!

Your not going to like the answer, but if you cant deal with it, Military life is not for you.
The answer, Your ship date changed.

Planned things are going to change constantly in your upcoming military life...Accept it or your set for failure.. you will be another unhappy one term service member that gets out with a bad attitude blaming all but the one actually at issue..the one in the mirror... the one that had unrealistic expectations and when those were not met, it MUST be the services fault...can't be me....

The 1st step is to accept plans are just that plans.....You will need to focus on what you are told NOW, not what was planned yesterday, or even a hour ago. Things change beyond your control, or control of the folks telling you things changed.... and there is NOTHING you can do about it, so get mad if you want but there is no gain in that..
If the plan was turn left at the end of the walk way but the NCO at the end is telling you to turn right..turn right and keep moving forward.

Moving your ship date earlier may cause you issues with your planning.... be it clearing out of your leased apartment, doctors appointment, getting married (I hope not) ect..if it is something you feel you just can not budge on.. go see your recruiter and work something out.

Welcome to the military my friend.