Whether you are calculating a project deadline, an expiration date, or a personal milestone, our Date Calculator simplifies the process of adding or subtracting time from any specific calendar day.
How the Calculation Works
To calculate a date, you must provide a starting point and the duration of time you wish to move forward or backward. The tool processes months and years correctly, accounting for the varying number of days in months and leap years.
Years & Months: These are added chronologically to the calendar month.
Weeks & Days: These are calculated as 7-day increments and 24-hour cycles respectively.
Leap Years: The calculator automatically adjusts for February 29th during leap year cycles.
Practical Examples
Example 1 (Project Management): If a 90-day contract begins on January 1st, adding 90 days will result in April 1st (or March 31st in a leap year).
Example 2 (Legal Deadlines): If a notice period is 6 months from July 15, adding 6 months results in January 15 of the following year.
Example 3 (Retroactive Planning): If you need to know what date was exactly 45 days ago to verify a receipt, simply select "Subtract" and input 45 days.
function calculateNewDate() {
var startInput = document.getElementById('startDate').value;
var operation = document.getElementById('operation').value;
var y = parseInt(document.getElementById('years').value) || 0;
var m = parseInt(document.getElementById('months').value) || 0;
var w = parseInt(document.getElementById('weeks').value) || 0;
var d = parseInt(document.getElementById('days').value) || 0;
if (!startInput) {
alert("Please select a starting date.");
return;
}
var resultDate = new Date(startInput);
// Adjust for Timezone offset to ensure local date consistency
resultDate.setMinutes(resultDate.getMinutes() + resultDate.getTimezoneOffset());
if (operation === 'add') {
resultDate.setFullYear(resultDate.getFullYear() + y);
resultDate.setMonth(resultDate.getMonth() + m);
resultDate.setDate(resultDate.getDate() + (w * 7) + d);
} else {
resultDate.setFullYear(resultDate.getFullYear() – y);
resultDate.setMonth(resultDate.getMonth() – m);
resultDate.setDate(resultDate.getDate() – (w * 7) – d);
}
var options = { weekday: 'long', year: 'numeric', month: 'long', day: 'numeric' };
var formattedDate = resultDate.toLocaleDateString(undefined, options);
document.getElementById('finalDate').innerText = formattedDate;
// Numerical format display
var dd = String(resultDate.getDate()).padStart(2, '0');
var mm = String(resultDate.getMonth() + 1).padStart(2, '0');
var yyyy = resultDate.getFullYear();
document.getElementById('dateDetails').innerText = "Numerical Format: " + mm + "/" + dd + "/" + yyyy;
document.getElementById('resultArea').style.display = 'block';
}
// Set default date to today
window.onload = function() {
var today = new Date();
var dd = String(today.getDate()).padStart(2, '0');
var mm = String(today.getMonth() + 1).padStart(2, '0');
var yyyy = today.getFullYear();
document.getElementById('startDate').value = yyyy + '-' + mm + '-' + dd;
};