The way in which we divide the day into hours and minutes, as well as the structure and length of the yearly calendar, owes much to pioneering developments in ancient Egypt.
Since Egyptian life and agriculture depended upon the annual flooding of the Nile, it was important to determine when such floods would begin. The early Egyptians noted that the beginning of akhet (inundation) occurred at the helical rising of a star they called Serpet (Sirius). It has been calculated that this sidereal year was only 12 minutes longer than the mean tropical year which influenced the flooding, and this produced a difference of only 25 days over the whole of Ancient Egypt's recorded history!
Ancient Egypt was run according to three different calendars. The first was a lunar calendar based on 12 lunar months, each of which began on the first day in which the old moon crescent was no longer visible in the East at dawn. (This is most unusual since other civilizations of that era are known to have started months with the first siting of the new crescent!) A thirteenth month was intercalated to maintain a link to the helical rising of Serpet. This calendar was used for religious festivals.
The second calendar, used for administrative purposes, was based on the observation that there was usually 365 days between the helical rising of Serpet. This civil calendar was split into twelve months of 30 days with an additional five epagomenal days attached at the end of the year. These additional five days were considered to be unlucky. Although there is no firm archaeological evidence, a detailed back calculation suggests that the Egyptian civil calendar dates back to c. 2900 BCE.
This 365 day calendar is also known as a wandering calendar, from the Latin name annus vagus since it slowly gets out of synchronization with the solar year. (Other wandering calendars include the Islamic year.)
A third calendar, which dates back at least to the fourth century BCE was used to match the lunar cycle to the civil year. It was based on a period of 25 civil years which was approximately equal 309 lunar months.
An attempt to reform the calendar to include a leap year was made at the beginning of the Ptolemetic dynasty (Decree of Canopus, 239 BCE), but the priesthood was too conservative to allow such a change. This pre-dates the Julian reform of 46 BCE which Julius Caesar introduced on the advise of the Alexandrian astronomer Sosigenese. Reform did, however, come after the defeat of Cleopatra and Anthony by the Roman General (and soon to be Emperor) Augustus in 31 BCE. In the following year the Roman senate decreed that the Egyptian calendar should include a leap year -- although the actual change to the calendar didn't occur until 23 BCE.