JavaScript Date Set Methods

Common Set Methods

let date = new Date();
date.setFullYear(2025);
date.setMonth(11);    // December (0-based)
date.setDate(25);     // Christmas!

let date = new Date();
date.setHours(14);
date.setMinutes(30);
date.setSeconds(0);

let date = new Date();
date.setTime(1640995200000); // Set to specific timestamp
date.setMilliseconds(500);   // Set milliseconds

let date = new Date();
date.setUTCHours(12);
date.setUTCMinutes(0);
date.setUTCSeconds(0);

🔹 Basic Set Methods

Here are the most commonly used set methods:

let date = new Date("2024-01-01 00:00:00");
console.log("Original:", date.toString());

// Set date components
date.setFullYear(2025);
date.setMonth(2);        // March (0-based)
date.setDate(15);

// Set time components
date.setHours(14);
date.setMinutes(30);
date.setSeconds(45);
date.setMilliseconds(500);

console.log("Modified:", date.toString());

🔹 Setting Multiple Values at Once

Some set methods can accept multiple parameters:

let date = new Date();

// setFullYear(year, month, day)
date.setFullYear(2024, 11, 25); // December 25, 2024

// setHours(hours, minutes, seconds, milliseconds)
date.setHours(9, 30, 0, 0); // 9:30:00.000 AM

// setMonth(month, day)
date.setMonth(0, 1); // January 1st

console.log("New Year:", date.toString());

🔹 Date Arithmetic with Set Methods

Use set methods to add or subtract time periods:

let date = new Date("2024-03-15");

// Add 30 days
date.setDate(date.getDate() + 30);
console.log("30 days later:", date.toDateString());

// Add 3 months
date.setMonth(date.getMonth() + 3);
console.log("3 months later:", date.toDateString());

// Add 1 year
date.setFullYear(date.getFullYear() + 1);
console.log("1 year later:", date.toDateString());

// Subtract 2 hours
date.setHours(date.getHours() - 2);
console.log("2 hours earlier:", date.toString());

🔹 UTC Set Methods

Set UTC (Universal Time) values instead of local time:

let date = new Date();

// Set local time
date.setHours(12, 0, 0, 0);
console.log("Local time:", date.toString());

// Set UTC time
date.setUTCHours(12, 0, 0, 0);
console.log("UTC time:", date.toString());

// All UTC set methods
date.setUTCFullYear(2024);
date.setUTCMonth(11);      // December
date.setUTCDate(25);       // Christmas
date.setUTCHours(0);       // Midnight UTC
date.setUTCMinutes(0);
date.setUTCSeconds(0);

console.log("Christmas UTC:", date.toString());

🔹 Practical Examples

Real-world examples using set methods:

// Set to beginning of day
function setToStartOfDay(date) {
    date.setHours(0, 0, 0, 0);
    return date;
}

let today = new Date();
console.log("Start of day:", setToStartOfDay(new Date(today)));

// Set to end of day
function setToEndOfDay(date) {
    date.setHours(23, 59, 59, 999);
    return date;
}

console.log("End of day:", setToEndOfDay(new Date(today)));

// Set to first day of month
function setToFirstDayOfMonth(date) {
    date.setDate(1);
    return date;
}

console.log("First of month:", setToFirstDayOfMonth(new Date(today)));

// Set to last day of month
function setToLastDayOfMonth(date) {
    date.setMonth(date.getMonth() + 1, 0); // Day 0 = last day of previous month
    return date;
}

console.log("Last of month:", setToLastDayOfMonth(new Date(today)));

// Create a specific date and time
function createDateTime(year, month, day, hour, minute) {
    let date = new Date();
    date.setFullYear(year);
    date.setMonth(month - 1); // Convert to 0-based
    date.setDate(day);
    date.setHours(hour, minute, 0, 0);
    return date;
}

let meeting = createDateTime(2024, 12, 25, 14, 30);
console.log("Meeting time:", meeting.toString());

🔹 Important Notes