The time system in which mass balance is determined by conducting field surveys on fixed calendar dates. The fixed date representing the start of the mass-balance year is usually at the start of the local hydrological year. To determine seasonal balances, a fixed date is chosen to represent the mean date of the end of the accumulation season. Due to logistical constraints it is often impossible to conduct field surveys on these exact dates. Therefore the data need to be corrected, which is often done by estimating ablation and accumulation between the survey date and the fixed date using meteorological data from a nearby weather station or a database of upper-air measurements. See also measurement year, stratigraphic system, floating-date system, combined system.