“Base year entitlement” means that amount determined to be the average for the approved reimbursement claims of each local agency or school district for the three preceding fiscal years adjusted by the change in the deflator. A base year entitlement shall not include any nonrecurring or initial startup costs incurred by a local agency or school district in any of those three fiscal years. For those mandates which become operative on January 1 of any year, the amount of the “approved reimbursement claim” for the first of the three years may be computed by annualizing the amount claimed for the six-month period of January through June in that first year, excluding nonrecurring or startup costs.