Hi Dazmond,
Glad you got this sorted

You do have to be a bit careful with the "Change Due" button as it is rather powerful! But as we figured out discussing this on the CP forum, you must have selected the "Change all jobs to 5/12/14" option, which of course will move ALL your jobs to the selected date. The system will default to the other option (the one you wanted) which was to "Push jobs back 18 days". If you'd kept it on this option it would have moved all the jobs back 18 days relative to their current due dates (thereby preserving their individual schedules) but I think you've figured that out now! Worth bearing in mind if this did happen again in the future, I take hourly backups, so if you make an error like this I can usually revert things for you.
Regarding whether it should be "push back" or "bring forward" it's up to debate really

You are pushing them back, i.e. pushing back the date when you'll do them (like when you "push back" a meeting), but I guess you're also bringing them forward i.e. into the future? Ah, you can look at it either way, but if the date is in the future then I don't think it can be misinterpreted?
Eco - yes good point, the planner retains your selection (I thought that would be handy) but maybe it shouldn't? I'll give that some thought...
Rob