The time span, equal or approximately equal in duration to one calendar year, to which the Annual mass balance in any time system refers. In the stratigraphic system the Annual mass balance is the change of mass during the period between formation of two successive minima in the sequence of Annual cycles of mass growth and decline. These minima are usually reached at different times in successive years, and the duration of the mass-balance year may therefore vary irregularly and substantially in duration from year to year. Point mass balances can be determined unambiguously in the stratigraphic system, but glacier-wide determinations require the assumption that the diachronous character of the summer surface can be neglected. In the fixed-date system the first day of the mass-balance year is always on the same calendar date, which is typically chosen to coincide with the start of the local hydrological year, for example 1 October in the mid-latitudes of the Northern Hemisphere or 1 April in the mid-latitudes of the Northern Hemisphere, or sometimes with the average date of minimum Annual mass. The mass-balance year is 365 (or 366) days long. In the floating-date system the mass-balance year is defined by the calendar dates of the two successive surveys, which may vary from year to year and may or may not be 365 (or 366) days apart.