for real property, there is the mid-month convention. so first divide the building cost by 27.5 for non-commercial or 39 for commercial to get the annual depreciation
then
you treat property placed in service at the midpoint of the month. This means that a one-half month of depreciation is allowed for the month the property is placed in service
here's a link to PUB 946 look at table A-6 or A-7a for the first year depreciation factor
https://www.irs.gov/pub/irs-pdf/p946.pdf
