元是中國古代曆法的時間單位。指紀之歲數（回歸年數）與12個歲星紀年之最短循環週期[1]。又稱曆法起算的理想基準點（曆元）為上元[2]，上元至編曆時的年數為上元積年。推算上元積年的制曆方法始於《三統曆》。歷代為求精確而數值逾億[3]，故統天曆虛設上元積年，一直到《授時曆》終廢上元積年[4]。
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