The time system in which the determination of mass balance is based on the identification of successive Annual minima, and for seasonal balances Annual maxima also, in the mass of the glacier or a part of the glacier. In field work, Annual mass balance is determined by the detection of two successive summer surfaces, usually at individual observation sites. In the ablation zone, the earlier summer surface has disappeared by the time the later one is observed, but its vertical position is known from earlier observations. For seasonal balances, it is not possible to determine the Annual maximum of mass with a single field survey that can be scheduled to coincide only roughly with the expected date of the maximum. Thus, in the stratigraphic system, seasonal balances by the glaciological method are actually measured in a combined system. Continuously recording sensors, such as snow pillows and sonic rangers, can yield accurate stratigraphic-system estimates of seasonal balances at single points, but they are not in wide use. The Annual extrema of mass may be reached at different times at different sites on the glacier. Glacier-wide balances in the stratigraphic system can be determined rigorously only by accurate spatially-distributed modeling or by gravimetric methods. Determinations based on field measurements must assume that the diachronous character of the summer surface can be neglected. The duration of the mass-balance year varies in the stratigraphic system.