Surface ¹⁴N abundance as a function of stellar age of our model 20 M_⊙ star with initial equatorial rotational velocity of 300 km s^-1 produced by the stellar evolution code BEC. The surface nitrogen abundance deviates between the two otherwise identical model sequences, with different fixed timesteps δt = 0.001 Myr (black dashed) and δt = 0.01 Myr (black dotted). The main-sequence lifetime is also different by about 10% between both model sequences. Numerical smoothing of the gradient (solid lines) enhances mixing of nitrogen to the surface. Models with higher resolution (blue lines) have a larger mean molecular weight gradient at the boundary of burning region which prohibits mixing. For models without fixed timesteps, the timesteps δt are adjusted by BEC code automatically and are of the order of 0.001 Myr.