How Age Is Computed
The standard approach (used here and on most birthday cards) calculates whole years between two dates: if the month-day of the reference date is on or after the month-day of birth, you've completed that year; otherwise subtract one. Remainder months and days are computed similarly. Leap years are handled correctly ??someone born on Feb 29 ages by one year on Feb 28 in non-leap years (or Mar 1, depending on jurisdiction). This calculator uses the Feb 28 convention.
Inclusive vs. Exclusive
"From Mon to Wed" can mean either 2 days (Wed ??Mon) or 3 days (Mon, Tue, Wed). The first is the standard arithmetic difference (exclusive end); the second is common when you're counting "how many days am I staying". The Date Difference tab supports both.
Month and Year Arithmetic Is Lossy
Adding months is ambiguous when the start date doesn't exist in the target month. Jan 31 + 1 month ??Feb 28 (or Feb 29 in leap years); the convention is to clamp to the last day of the month. This is what JavaScript's Date object does and what this tool follows.
Weekdays
"Weekday" here means Monday through Friday. The Weekday Count tab also supports counting only weekends or all days. Public holidays are NOT excluded ??for business-day calculations that account for U.S. federal holidays or your country's calendar, treat this number as an upper bound.