There seems to be some confusion regarding the IDL. I have gone through Wikipedia, but that too does not make any sense at all. We know that the countries west of us are behind our time. India is about 11 hours ahead of the US(depending on their time zones), so if its Friday morning here 7 am here in India, its Thursday evening 8 pm over there.
Now if we consider New Zealand and US(Hawaii), there's nearly 23 hours of time difference between them, but still US is behind the New Zealand time, isn't that correct. For ex- Friday morning 7 am in NZ would be Thursday morning 8 am in Hawaii. Now take two people standing on opposite sides of the IDL. One will be in the western hemisphere and the other east. The guy on the east will be a whole day(24hours) ahead of the guy on the west.
Now take a big step and cross the line, and there you are in the western hemisphere with a date subtracted, not added. This is just my logic, please feel free to correct me if I am wrong.