According to hindu calander, we must held every festival on the exact date and time which is mentioned in the scriptures.

In Puri every year, Temple organizes Jaggannath Rathyatra on the very particular day and time.

According to ISKCON members and gurus Rathyatra is the only chance when they do get together and collect fund on the name of KC.

Why dont you (ISKCON people) celebrate Janmastami any other day? Why dont you celebrate holi or diwali or Radhastami any other day? Why dont you celebrate Prabhupad disappearance thru out the year in the various cities?
Infact, once I personally went to Watford temple in UK. and ISKCON temple was celebrating Dusherra 5 days later. When I asked them. they said, "this time Dusheera came on working day. Not many people would have come, so we changed the date"
Are you runing the Temple of Lord? Where is the Gaudiya Mood (Bhaav)? If you guys say, you are the followers of SIX Goswamis and Seven Temples, do anyone from there break the rules? Temple should take care His Lord first because it is HIS residence. You are not allowed to change the dates because devotees wont participate in the festival.
You guys are actually ruining our culture on the name of KC. You guys should read lawbooks of our Sanatan Dharma. If your Prabhupad did anything, it doesnt mean we should also follow him and change our scriptures' quotations and rules. He(prabhupad) was only the guru not the GOD.

Puri priests are right with their points. They are not stopping you. They just want you to celebrate Rathyatra on the same day and time (according to the scriptures)