A "how many days" calculator is a tool used to calculate the number of calendar days between two given dates. These calculators also often take into account leap years automatically and may offer options to include or exclude the end date. Some advanced versions can break down the count into weekdays, weekends, holidays, and even express the duration in weeks, months, or years. To use such a calculator:

• Enter the start date and end date.
• Choose whether to include the last day in the count.
• The calculator then outputs the number of days between those dates, considering all calendar variations like leap years.

This is useful for finding out the duration between events or counting down to a future date easily without manual calculation.