LANLGeoMag is a C-based library of magnetic-field models and tools for computing quantities relevant to geophysical and geospace research that rely on these models. More generally, the library provides functionality to perform high precision transforms between time standards and coordinate systems. The primary uses of this library are coordinate conversions, magnetic field evaluation, magnetic field line tracing, drift shell tracing, calculation of diffusion coefficients and bounce-averaging of quantities in arbitrary magnetic fields.