This study assesses the impact of the time resolution and design day on the estimated lifetime of the molten-salt external tubular receiver of a solar power tower, one of the most damaged components of these facilities, considering operation under clear conditions. A global analysis is performed by first determining the heliostat field aiming strategy; the receiver operation limits are set to keep a low enough film temperature and to avoid the stress reset. The former prevents excessive corrosion rates of the tubes while the latter assures the global stress relaxation, which significantly reduces their damage during the receiver cyclic operation. Time steps of 60, 30, 15, 5 and 1 min are tested considering the spring equinox design day, as well as only solar noon conditions. The latter significantly underpredicts the receiver lifetime with respect to the 1-min case, being early discarded. The lifetime in the most damaged panel is underestimated over 18% and 16% using the 60- and 30-minute time steps, dropping to 2.57% using the 5-min time step at a reasonable computational cost. Finer resolutions enable more precise aiming strategy selection, decreasing the receiver peak fluxes. Lastly, a set of 8 representative days for the year, equally spaced in solar height, is more accurate than using the spring equinox alone, which results in an underestimate of the receiver lifetime that may be overly conservative. The summer solstice is the least-damaging day, with the lifetime decreasing as approaching the winter one, as long as the storage tank is filled. This research is partially funded by the scholarship "Ayudas para la formación del profesorado universitario" (FPU-02361) awarded by the Spanish Ministerio de Educación, Cultura y Deporte (MECD), the Spanish government under the project RTI2018-096664-B-C21 (MICINN/FEDER, UE) and the call "Programa de apoyo a la realización de proyectos interdisciplinares de I + D para jóvenes investigadores de la Universidad Carlos III de Madrid 2019-2020", under the projects RETOrenovable-CM-UC3M and ZEROGASPAIN-CM-UC3M, funded on the frame of the Convenio Plurianual Comunidad de Madrid- Universidad Carlos III de Madrid.