Don't just make up when it was placed in service.
If it was actually placed in service on January 1st, enter January 1st and do it correctly. you can't just say December 31st just because you want a full year of depreciation. If it actually was placed in service in December 31st, then you should have had a 1/2 month of depreciation on your 2017 tax return, which lowers your Basis of the property. In that case you may consider amending 2017 because when the property is sold at a gain, you WILL pay tax on the depreciation, regardless if you took that 1/2 month of depreciation or not.
Again, don't just make up numbers; do it right.
