Starless cores are the potential sites for star and planet formation. Although we know gravity plays the main role during evolution, the details, in particular the timescale, are not yet well understood. The lifetime suggested by different scenarios could vary by more than a factor of ten. With time-dependent chemical analysis, measuring the chemical timescale of the cores allows us to infer possible evolutionary scenarios. We determine the density, temperature, and molecular abundance profiles of two nearby low-mass starless cores, L1512 and L1498, with dust extinction measurements from near-infrared observations and non-local thermal equilibrium radiative transfer with single-dish radio observations (H<sub>2</sub>D<sup>+</sup> as well as deuterated N-bearing tracers with IRAM30m, JCMT, and GBT). Then we perform chemical modeling of the two targets to measure their chemical timescales using deuterium fractionation as a chemical clock. We find that L1512 is chemically evolved while L1498 is chemically young. This might imply that the magnetic field is stronger in L1512 than in L1498. Consequently, ambipolar diffusion may have slowed the contraction of L1512 or even halted it to the present state.