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The time duration from 12am (midnight) to 9pm is exactly 21 hours, which equals 1260 minutes or 75600 seconds.
12am (midnight) to 9pm spans 21 hours. These early morning hours are often a quiet time when most people are asleep.
The calculator above automatically sets the times to 12am (00:00) and 9pm (21:00), but you can adjust the date or add seconds for more specific calculations.