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Astronomical year numbering is based on AD/CE year numbering, but follows normal decimal integer numbering more strictly. Thus, it has a year 0 and the years before that are designated with a minus sign '−'. The era designations AD/CE are dropped. So the year 1 BC(E) is numbered 0, the year 2 BC(E) is numbered −1, and in general the year n BC(E) is numbered (1−n). The numbers of AD/CE years are not changed, but AD/CE is not used, being replaced by either no sign or a positive sign. For normal calculation a number zero is often needed, here most notably when calculating the number of years in a period that spans the epoch; the end years need only be subtracted from each other.