What's the difference between period and duration?

The difference between period and duration is subtle but important, as both relate to time but emphasize different aspects. Duration means length of time something lasts, how long an event continues from start to finish, measured in units like hours, days, or years, focusing on elapsed time. For example, duration of movie is 2 hours, duration of project is 3 months. Period means a specific segment or interval of time with defined start and end points, often recurring or marked by events, focusing on when something occurs and its place in calendar. For example, trial period is January to March, billing period is monthly, historical period like Renaissance. Duration answers how long, period answers when or what segment. In physics, period is time for one cycle of repeating event, while duration is total time event lasts. In finance, duration measures sensitivity to interest rates, period is time frame like quarterly period. In roofing, duration is shingle brand name, period is time period of warranty. In grammar, period is punctuation mark. In work, work period is shift, work duration is length of shift. So period is container or segment of time, duration is length of that container. For example, work period might be 9 AM to 5 PM, duration is 8 hours. Period can be recurring, duration is measurement. Both relate to time but period emphasizes interval, duration emphasizes length.