<P> An astronomical calendar is based on ongoing observation; examples are the religious Islamic calendar and the old religious Jewish calendar in the time of the Second Temple . Such a calendar is also referred to as an observation - based calendar . The advantage of such a calendar is that it is perfectly and perpetually accurate . The disadvantage is that working out when a particular date would occur is difficult . </P> <P> An arithmetic calendar is one that is based on a strict set of rules; an example is the current Jewish calendar . Such a calendar is also referred to as a rule - based calendar . The advantage of such a calendar is the ease of calculating when a particular date occurs . The disadvantage is imperfect accuracy . Furthermore, even if the calendar is very accurate, its accuracy diminishes slowly over time, owing to changes in Earth's rotation . This limits the lifetime of an accurate arithmetic calendar to a few thousand years . After then, the rules would need to be modified from observations made since the invention of the calendar . </P> <Table> <Tr> <Td> </Td> <Td> This section's factual accuracy is disputed . Relevant discussion may be found on Talk: Calendar . Please help to ensure that disputed statements are reliably sourced . (June 2017) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This section's factual accuracy is disputed . Relevant discussion may be found on Talk: Calendar . Please help to ensure that disputed statements are reliably sourced . (June 2017) (Learn how and when to remove this template message) </Td> </Tr>

Where do we find the clock and calender in our computer