retained earnings

Banjo Co. purchase a building on 30 June 20X8 for $1250,000. At acquisition, the useful life of the building was 50years. Depreciation is calculated on the straight line basis. 10years later, on 30 June 20Y8 when the carrying amount of the building was $1,000,000, the building was revaluated to $1,600,000. Banjo Co. has a policy of transferring the excess depreciation on revaluation from the revaluation surplus to retained earnings.

Assuming no further revaluation take place, what is the balance on the revaluation surplus at 30June 20Y9? Answer is $585,000

Sir, my question is that, there are 13 months but why is 12 months of excess depreciation is subtracted ? is it something to do with that it is subtracted yearly to show in income statement?