最簡單嘅曆法系統係以某一個參考日或者時間爲準，計算經過咗幾多個時間單位，好似儒略日同埋UNIX時間咁就係呢種系統，如果時間單位唔變嘅話，唯一可能嘅變化就係改參考日或者時間，另到計算嘅時間單位少啲，呢種曆法系統嘅換算只需要加法同減法。
The simplest calendar system is based on a certain reference day or time, counting how many time units have passed, such as Julian day and UNIX time is this kind of system, if the time unit does not change, the only possible change is to change the reference day or time, and the other time unit is a little less, the conversion of this calendar system only needs addition and subtraction.