In the present work, the time-dependent density functional theory (TD-DFT) is applied to study the electronic transitions which give rise to the near-ultraviolet-visible (near-UV-vis) optical absorptions of 3,4-dimethyl-N-{2-[10,15,20-tris-(3,5-di-tert-butyl-phenyl)-porphyrin-5-yl]-phenyl} benzamide (H2P-O34) and its zinc complex ZnP-O34. Optimizations of the molecular structures were performed using the GGA type Perdew-Burke-Ernzerhof exchange-correlation functional. Excitation energies and oscillator strengths were obtained from the TD-DFT calculations. Calculations were also carried out by using the BP and B3LYP functionals. A large number of electronic transitions are responsible for the optical absorptions in the near-UV-vis region. It was also found that distortions induced to the porphyrin ring by the substituents contribute < 10% to the red shifts of the absorption wavelengths of the substituted porphyrins with respect to the unsubstituted porphyrins. The energies of the lowest triplet states were calculated to be 1.44 eV for H2P-O34 and 1.61 eV for ZnP-O34.