Millisecond (ms)
Definition: A millisecond is one thousandth of a second.
History/origin: The millisecond grew in importance with telegraphy, industrial control, computing, and modern user-interface timing.
Current use: Milliseconds are common in software performance, gaming latency, response times, and many industrial or medical devices.
Day (day)
Definition: The day is the normal calendar time unit based on Earth's rotation. In converters, one day is treated as 24 hours or 86,400 seconds.
History/origin: Long before mechanical clocks, people tracked time by sunrise, sunset, and the repeating light-dark cycle that became the civil day.
Current use: Days are used for due dates, delivery windows, travel plans, medication intervals, streaks, reservations, payroll periods, and calendar-based scheduling.