There is no deadline for migration of the Windows Server 2012/R2 or SQL Server 2012 workloads to Azure. However, we recommend customers complete migration before the end of support date so that they do not miss any ESUs. If customers miss a year of ESUs coverage, they may buy coverage for previous years at the same time they buy coverage for a current period.
To find more information about end of support dates, refer to: Search Product and Services Lifecycle Information -- Microsoft Lifecycle | Microsoft Docs.