Time and Calendar - Duration of Time

Duration is the measurement of how long an event lasts. It is the time elapsed between the starting time and the finishing time. Visualize a timeline where the start time is a point on the left and the end time is a point on the right; the length of the line connecting them represents the duration.

The 12-hour clock system uses $\text{a.m.}$ (Ante Meridiem) for the time from midnight to noon and $\text{p.m.}$ (Post Meridiem) for the time from noon to midnight. Visualize a circular clock face where the hour hand completes two full circles every day—the first circle for $\text{a.m.}$ and the second for $\text{p.m.}$

To calculate the duration of an event, we find the difference between the end time and the start time. If the minutes in the end time are smaller than the start time, we borrow $1 \text{ hour}$ ($60 \text{ minutes}$) from the hour column. Imagine a vertical subtraction table with 'Hours' and 'Minutes' columns.

Time units are hierarchical: $60 \text{ seconds}$ make $1 \text{ minute}$, $60 \text{ minutes}$ make $1 \text{ hour}$, and $24 \text{ hours}$ make $1 \text{ day}$. Visualize a set of nesting boxes where $60$ tiny 'second' blocks fit into $1$ 'minute' box, and $60$ 'minute' boxes fit into $1$ 'hour' box.

A calendar helps track days, weeks, and months. There are $7 \text{ days}$ in a week and $12 \text{ months}$ in a year. Visualize a grid with $7$ columns representing the days of the week (Sunday to Saturday) and several rows representing the weeks in a month.

The number of days in months varies between $28$ and $31$. You can use the 'Knuckle Rule' to remember: knuckles represent months with $31 \text{ days}$ (January, March, May, July, August, October, December), while the gaps between knuckles represent months with $30 \text{ days}$ (April, June, September, November), except February.

A leap year occurs every $4$ years and has $366 \text{ days}$ instead of $365$. In a leap year, February has $29 \text{ days}$ instead of $28$. Imagine a calendar where an extra block is added to the end of February once every four years.