We are so used to using calendars that we do not often ask when they were first used and why they were used. One of the earliest places where calendars are known to have been in use is ancient Egypt. Egyptian agriculture was based on using the Nile flood water. The Nile floods at the same time every year but the cause of the flood is not connected with the weather in Egypt itself. It was therefore essential to be able to predict the time of the Nile flood in advance if advantage is to be made of the flood waters. However this is a special situation. For example when deciding when to prune my roses in the spring I could consult a calendar. However if I had no calendar I could easily find the right time of year for pruning the roses by observing when some other plant had reached a certain stage of development. In fact given the difference between one season and another this method is probably better than using a calendar anyway.