古埃及的历史相当悠久，历法几经转变，且不同学派有不同的说法。本条目描述的是获得较广泛认同的说法。

古埃及人分昼夜各占12「小时」，从日出到日落为昼，从日落到日出为夜。但在一年之中，昼夜的长度并非固定不变，因此一小时的长度会随着季节变化，白天的一小时和晚上的1小时，很多时候并不一样。

每月固定有30天，没有大、**之分。一年共有3季，每季4个月，因此名义上一年只有360天。但每年新年前都会加上5天，叫做Epagomenal Days，在这5天，他们会举行仪式庆祝新年。就这样，两个新年实际上相隔的日数便为365天。

The ancient civil Egyptian calendar had a year that was 365 days long. The year consisted of 12 months of 30 days each, plus five extra days (epagomenae, from Greek ἐπαγόμεναι) at the end of the year. The months were divided into three weeks (decans) of ten days each. Because the ancient Egyptian year was almost a quarter of a day shorter than the solar year and stellar events therefore "wandered" through the calendar, it has been referred to as the annus vagus, or "wandering year".