The growing interest in hydrogen as an alternative energy vector has raised new technological challenges, in particular regarding its storage. This has motivated increasing attention to ammonia as a hydrogen carrier. In parallel, the use of hydrogen-ammonia blends as combustible fuels has attracted significant interest, as such mixtures can be easier to handle in some applications than pure hydrogen, while still enabling carbon-free combustion.In this context, the present study focuses on modeling the ignition of arbitrary gaseous hydrogen-ammonia-air blends. First, the minimal chemical description required to accurately capture the ignition delay of these mixtures is identified, revealing three main ignition regimes. Ignition delay formulas are then derived for these regimes by extending methods previously developed for pure hydrogen and syngas. The resulting ignition time expressions are subsequently combined into a unified formulation, valid across a wide range of pressures, temperatures, and fuel compositions. Finally, modifications to a recently published passive scalar model for CFD tools are introduced so as to accurately predict ignition events in hydrogen-ammonia-air mixtures while reducing computational cost. Novelty and