I ran into a similar problem and ran out of space in my compostela. I collected one stamp per day in my compostela and two or more stamps per day in my journal. The Pilgrim Office accepted this as fulfilling their requirements.
I think if you were to just explain the situation and ask them to use your actual start date on the Certificate of Completion when you visit the Pilgrim Office they would be willing to do that. Seems like a small ask to me.