Most reporting entities must apply the new lease accounting standard (ASC 842/IFRS 16) with the fiscal year that starts in 2019. To be precise, ASC 842 requires implementation for fiscal years starting after Dec. 15, 2018, while IFRS 16 requires implementation for fiscal years starting Jan. 1, 2019, or later. Earlier implementation is permitted, though IFRS 16 permits implementation no earlier than implementation of IFRS 15, Revenue from Contracts with Customers. ASC 842 permits delayed implementation for privately-held entities, including not-for-profits that have not issued listed securities; such entities may implement in the fiscal year starting Dec. 15, 2021, and need not apply it until their year-end report (other entities must apply the new standard to quarterly or other interim reports). GASB 87 must be implemented in the fiscal year that starts in 2021.
Public entities using a calendar year fiscal year, then, should enter 1/1/2019.