Date Calculator

Calculating the difference between two dates or adding days to a date is a common task in project planning, age calculations, deadline tracking, and financial scheduling. The math behind date arithmetic involves handling variable month lengths, leap years, and timezone considerations.

This tool performs two core operations: finding the duration between two dates (in days, weeks, months, and years) and computing a new date by adding or subtracting a number of days from a start date. All calculations run in your browser with no data sent to any server.

Dates are typically entered in ISO 8601 format (YYYY-MM-DD), which is unambiguous and sorts correctly as text. Most modern date pickers and APIs use this format. The calculator also displays results in a human-readable form with day of week and ISO week number.

ISO week numbering defines week 1 as the week containing the first Thursday of the year. This means some years have 52 weeks and others 53. The week number is useful for scheduling, payroll, and reporting that operates on weekly cycles.

Date calculations depend on where you are. A date like '2024-01-15' represents midnight in your local timezone. When you cross timezone boundaries, the same moment in time can fall on different calendar dates in different places.

For most everyday use — project deadlines, age calculations, vacation planning — local dates are sufficient. For distributed teams or systems spanning multiple timezones, consider using UTC timestamps or a dedicated timezone converter.

Results include the day of the week (e.g., Monday, Tuesday) and the ISO week number. The day of week helps you quickly see whether a date falls on a weekend or weekday. Week numbers are used in many industries for planning and reporting.

When adding or subtracting days, the result shows the full date, day of week, and week number so you can verify the outcome and use it in scheduling or documentation.