The number of days between two dates can be calculated by subtracting the start date from the end date. If the time span crosses years, leap years must be taken into account. The calculation can exclude or include the end date, with including it adding 1 day to the result. To convert days into weeks, divide by 7, and for months and years, adjustments for different month lengths and leap years are necessary. Several online calculators can simplify this task, providing exact numbers of days, weeks, months, and even years between two given dates.