Temporal structures are integral dimensions of all social activity ranging from the simple act of two people walking down the street together to a nation organizing itself to repel an invasion. The calendar is one of the most important information technologies ever devised. The evolution of timekeeping procedures that allow for such activity spanned tens of thousands of years. People began to develop calendars when they began noting that certain significant events such as the reappearance of migrating herds of animals followed the occurrence of other specific events. The development of agriculture did not cause sedentary agricultural communities to develop more elaborate and precise calendars nor did the development of more precise calendars cause the development of agriculture. The information specialists who were provided with necessities of life by their communities were the timekeepers of agricultural communities. The ancient calendars did not measure durations. Many societies used sundials, hourglasses, and water clocks to specify units of duration.