JavaScript 时间戳完整指南

本文暂提供英文版，中文翻译正在完善中。

Introduction

JavaScript provides the Date object for working with dates and timestamps. Understanding how to properly use timestamps in JavaScript is essential for web development, data processing, and time-based applications. This guide covers timestamp fundamentals, conversions, timezone handling, and best practices.

Understanding JavaScript Timestamps

Unix Timestamp in JavaScript

JavaScript uses Unix timestamps in milliseconds since January 1, 1970, 00:00:00 UTC (the Unix epoch):

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TEXT
1JavaScript Timestamp = Milliseconds since 1970-01-01 00:00:00 UTC
2
3Example: 1704624000000
4  = 1704624000 seconds since epoch
5  = January 8, 2024, 12:00:00 PM UTC

Important: JavaScript uses milliseconds while many other systems use seconds. Always verify units when converting.

Current Timestamp

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JAVASCRIPT
1// Current timestamp in milliseconds
2const nowMs = Date.now();
3console.log(nowMs);  // e.g., 1704624000000
4
5// Alternative method
6const nowMs2 = new Date().getTime();
7console.log(nowMs2);  // Same as Date.now()
8
9// Convert to seconds
10const nowSec = Math.floor(Date.now() / 1000);
11console.log(nowSec);  // e.g., 1704624000

Creating Dates

Creating Date Objects

From Timestamp

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JAVASCRIPT
1// Create from milliseconds timestamp
2const dateFromMs = new Date(1704624000000);
3console.log(dateFromMs.toISOString());  // "2024-01-08T12:00:00.000Z"
4
5// Create from seconds timestamp (multiply by 1000)
6const dateFromSec = new Date(1704624000 * 1000);
7console.log(dateFromSec.toISOString());  // "2024-01-08T12:00:00.000Z"

From Date Components

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JAVASCRIPT
1// Create from year, month, day (months are 0-indexed!)
2const date1 = new Date(2024, 0, 8);  // January 8, 2024
3console.log(date1.toISOString());  // "2024-01-08T00:00:00.000Z"
4
5// Create with time
6const date2 = new Date(2024, 0, 8, 14, 30, 0);
7console.log(date2.toISOString());  // "2024-01-08T14:30:00.000Z"
8
9// Create from string (ISO 8601 recommended)
10const date3 = new Date('2024-01-08T14:30:00.000Z');
11console.log(date3.toISOString());  // "2024-01-08T14:30:00.000Z"

Common Mistake: JavaScript months are 0-indexed (0 = January, 11 = December)!

Current Date and Time

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JAVASCRIPT
1const now = new Date();
2
3console.log(now.toISOString());        // "2024-01-08T12:34:56.789Z"
4console.log(now.toString());           // "Mon Jan 08 2024 07:34:56 GMT-0500"
5console.log(now.toDateString());      // "Mon Jan 08 2024"
6console.log(now.toTimeString());      // "07:34:56 GMT-0500"
7console.log(now.toLocaleString());     // "1/8/2024, 7:34:56 AM"

Getting Date Components

Extracting Date Parts

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JAVASCRIPT
1const date = new Date(2024, 0, 8, 14, 30, 45, 123);
2
3// Get components
4console.log(date.getFullYear());      // 2024
5console.log(date.getMonth());        // 0 (January - 0-indexed!)
6console.log(date.getDate());         // 8 (day of month)
7console.log(date.getHours());        // 14 (hour 0-23)
8console.log(date.getMinutes());      // 30 (minutes 0-59)
9console.log(date.getSeconds());      // 45 (seconds 0-59)
10console.log(date.getMilliseconds()); // 123 (milliseconds 0-999)
11
12// Get day of week (0 = Sunday, 6 = Saturday)
13console.log(date.getDay());          // 1 (Monday)
14
15// Get UTC components
16console.log(date.getUTCFullYear()); // 2024
17console.log(date.getUTCHours());   // 19 (if local is 14:30)

Relative Time Components

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JAVASCRIPT
1const date = new Date(2024, 0, 8);
2
3// Get timezone offset (in minutes)
4const offset = date.getTimezoneOffset();
5console.log(offset);  // e.g., -300 (UTC-5 = -5 * 60)
6
7// Get timestamp
8const timestamp = date.getTime();
9console.log(timestamp);  // e.g., 1704672000000 (milliseconds)
10
11// Get ISO 8601 string
12const isoString = date.toISOString();
13console.log(isoString);  // "2024-01-08T00:00:00.000Z"

Setting Date Components

Modifying Dates

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JAVASCRIPT
1const date = new Date(2024, 0, 8);
2
3// Set components (returns timestamp)
4date.setFullYear(2025);
5date.setMonth(11);      // December
6date.setDate(25);
7date.setHours(18);
8date.setMinutes(30);
9date.setSeconds(45);
10date.setMilliseconds(0);
11
12console.log(date.toISOString());  // "2025-12-25T18:30:45.000Z"
13
14// Set UTC components
15date.setUTCFullYear(2025);
16date.setUTCHours(12);

Important: Set methods return the timestamp, not the Date object. Chain carefully:

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JAVASCRIPT
1// Correct: Use same date object
2const date = new Date(2024, 0, 8);
3date.setDate(15);
4date.setMonth(5);  // June 15, 2024
5
6// WRONG: Creates new date, loses changes
7const date = new Date(2024, 0, 8);
8new Date(date).setDate(15);  // Original date unchanged!

Date Arithmetic

Adding and Subtracting Time

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JAVASCRIPT
1const date = new Date(2024, 0, 8);
2
3// Add 7 days
4date.setDate(date.getDate() + 7);
5console.log(date.toISOString());  // "2024-01-15T00:00:00.000Z"
6
7// Add 3 hours (3600 * 1000 ms)
8date.setTime(date.getTime() + (3 * 3600 * 1000));
9console.log(date.toISOString());  // "2024-01-15T03:00:00.000Z"
10
11// Subtract 1 month
12date.setMonth(date.getMonth() - 1);
13console.log(date.toISOString());  // "2023-12-15T03:00:00.000Z"
14
15// Add 1 year
16date.setFullYear(date.getFullYear() + 1);
17console.log(date.toISOString());  // "2024-12-15T03:00:00.000Z"

Best Practice: Use timestamp arithmetic for precision:

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JAVASCRIPT
1// Add 1 hour in milliseconds (60 * 60 * 1000)
2const oneHour = 60 * 60 * 1000;
3const newDate = new Date(date.getTime() + oneHour);

Time Differences

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JAVASCRIPT
1const date1 = new Date(2024, 0, 8);
2const date2 = new Date(2024, 0, 15);
3
4// Difference in milliseconds
5const diffMs = date2 - date1;
6console.log(diffMs);  // 604800000 (7 days in ms)
7
8// Convert to seconds, minutes, hours, days
9const diffSec = Math.floor(diffMs / 1000);
10const diffMin = Math.floor(diffMs / (1000 * 60));
11const diffHours = Math.floor(diffMs / (1000 * 60 * 60));
12const diffDays = Math.floor(diffMs / (1000 * 60 * 60 * 24));
13
14console.log(`${diffDays} days`);  // "7 days"
15console.log(`${diffHours} hours`);  // "168 hours"

Formatting Dates

Built-in Formatting

toLocaleString Family

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JAVASCRIPT
1const date = new Date(2024, 0, 8, 14, 30, 45);
2
3// Locale-specific formats
4console.log(date.toLocaleDateString());      // "1/8/2024"
5console.log(date.toLocaleTimeString());      // "2:30:45 PM"
6console.log(date.toLocaleString());         // "1/8/2024, 2:30:45 PM"
7
8// With locale options
9console.log(date.toLocaleDateString('en-US', {
10  weekday: 'long',
11  year: 'numeric',
12  month: 'long',
13  day: 'numeric'
14}));
15// "Monday, January 8, 2024"
16
17console.log(date.toLocaleDateString('zh-CN', {
18  year: 'numeric',
19  month: 'long',
20  day: 'numeric'
21}));
22// "2024年1月8日"

International Formats

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JAVASCRIPT
1const date = new Date(2024, 0, 8);
2
3// ISO 8601 (always UTC)
4console.log(date.toISOString());  // "2024-01-08T00:00:00.000Z"
5
6// UTC string
7console.log(date.toUTCString());  // "Sun, 07 Jan 2024 00:00:00 GMT"
8
9// Date string (local time)
10console.log(date.toString());  // "Mon Jan 08 2024 00:00:00 GMT-0500"

Best Practice: Always use toISOString() for API calls and database storage to avoid timezone issues.

Custom Formatting

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JAVASCRIPT
1// Manual formatting (use Intl.DateTimeFormat for i18n)
2function formatDate(date) {
3  const year = date.getFullYear();
4  const month = String(date.getMonth() + 1).padStart(2, '0');
5  const day = String(date.getDate()).padStart(2, '0');
6  const hours = String(date.getHours()).padStart(2, '0');
7  const minutes = String(date.getMinutes()).padStart(2, '0');
8  const seconds = String(date.getSeconds()).padStart(2, '0');
9
10  return `${year}-${month}-${day} ${hours}:${minutes}:${seconds}`;
11}
12
13console.log(formatDate(new Date(2024, 0, 8, 14, 30, 45)));
14// "2024-01-08 14:30:45"

Recommendation: Use libraries like date-fns, luxon, or day.js for complex formatting instead of manual formatting.

Timezone Handling

Working with Timezones

Getting Timezone Info

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JAVASCRIPT
1const date = new Date();
2
3// Get timezone offset (in minutes)
4const offset = date.getTimezoneOffset();
5console.log(offset);  // e.g., -300 (UTC-5) or 300 (UTC+5)
6
7// Convert offset to hours
8const offsetHours = offset / 60;
9console.log(offsetHours);  // -5 or 5
10
11// Get timezone name (browser-specific)
12const timezone = Intl.DateTimeFormat().resolvedOptions().timeZone;
13console.log(timezone);  // e.g., "America/New_York"

Converting Between Timezones

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JAVASCRIPT
1const date = new Date(2024, 0, 8, 14, 30, 0);
2
3// Convert to different timezone using toLocaleString
4const nyTime = date.toLocaleString('en-US', {
5  timeZone: 'America/New_York',
6  timeZoneName: 'short'
7});
8console.log(nyTime);  // "1/8/2024, 9:30:00 AM EST"
9
10const tokyoTime = date.toLocaleString('en-US', {
11  timeZone: 'Asia/Tokyo'
12});
13console.log(tokyoTime);  // "1/8/2024, 11:30:00 PM"
14
15// Extract time components in specific timezone
16const tokyoDate = new Date(date.toLocaleString('en-US', {
17  timeZone: 'Asia/Tokyo'
18}));
19console.log(tokyoDate.getHours());  // 23 (11 PM)

Best Practice: Always work with UTC internally and convert to local timezone only for display.

UTC vs Local Time

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JAVASCRIPT
1const date = new Date(2024, 0, 8, 14, 30, 0);
2
3// Local time methods
4console.log(date.getHours());     // Local hour (e.g., 14 for EST)
5console.log(date.getTime());    // Timestamp (same regardless of timezone)
6
7// UTC methods
8console.log(date.getUTCHours()); // UTC hour (e.g., 19 for EST)
9console.log(date.getUTCDate());  // UTC day of month

Parsing Dates

Parsing Date Strings

Using Date Constructor

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JAVASCRIPT
1// ISO 8601 (recommended)
2const isoDate = new Date('2024-01-08T14:30:00.000Z');
3console.log(isoDate.toISOString());  // "2024-01-08T14:30:00.000Z"
4
5// Short date format (browser-dependent, avoid!)
6const shortDate = new Date('01/08/2024');
7console.log(shortDate);  // May be Jan 8 or Aug 8 depending on locale
8
9// RFC 2822 format
10const rfcDate = new Date('Mon, 08 Jan 2024 14:30:00 GMT');
11console.log(rfcDate.toISOString());  // "2024-01-08T14:30:00.000Z"

Problem: Date parsing is inconsistent across browsers and locales.

Manual Parsing

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JAVASCRIPT
1// Parse ISO 8601 string manually
2function parseISOString(isoString) {
3  const date = new Date(isoString);
4  if (isNaN(date.getTime())) {
5    throw new Error('Invalid date string');
6  }
7  return date;
8}
9
10// Parse custom format (YYYY-MM-DD)
11function parseCustomDate(dateString) {
12  const [year, month, day] = dateString.split('-').map(Number);
13  return new Date(year, month - 1, day);  // months are 0-indexed
14}
15
16console.log(parseCustomDate('2024-01-08').toISOString());
17// "2024-01-08T00:00:00.000Z"

Recommendation: Use libraries like date-fns/parse or luxon/DateTime.fromISO for reliable parsing.

Validating Dates

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JAVASCRIPT
1// Check if date is valid
2function isValidDate(date) {
3  return date instanceof Date && !isNaN(date.getTime());
4}
5
6// Examples
7const validDate = new Date(2024, 0, 8);
8const invalidDate = new Date('invalid');
9
10console.log(isValidDate(validDate));    // true
11console.log(isValidDate(invalidDate));  // false
12console.log(isNaN(invalidDate.getTime()));  // true (NaN)

Common Pitfalls

Month Indexing

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JAVASCRIPT
1// Common mistake: Using 1-12 for months
2const date = new Date(2024, 12, 8);  // January 2025!
3console.log(date.toISOString());  // "2025-01-08T00:00:00.000Z"
4
5// Correct: Use 0-11 for months
6const date = new Date(2024, 11, 8);  // December 2024
7console.log(date.toISOString());  // "2024-12-08T00:00:00.000Z"

Timezone Confusion

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JAVASCRIPT
1const date = new Date(2024, 0, 8, 0, 0, 0);
2
3// These produce different values!
4console.log(date.getTime());     // 1704672000000 (same)
5console.log(date.toString());     // "Mon Jan 08 2024 00:00:00 GMT-0500"
6console.log(date.toISOString()); // "2024-01-08T05:00:00.000Z"
7
8// Midnight EST = 5:00 AM UTC

Problem: Creating dates without timezone info assumes local time.

String Parsing Inconsistencies

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JAVASCRIPT
1// Different browsers may parse differently
2const date1 = new Date('2024-01-08');      // Might be Jan 8 or Jan 1
3const date2 = new Date('2024/01/08');      // Might be parsed differently
4const date3 = new Date('01-08-2024');      // Very ambiguous!
5
6// Always use ISO 8601
7const date = new Date('2024-01-08T00:00:00Z');
8console.log(date.toISOString());  // Consistent across all browsers

Math Overflow

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JAVASCRIPT
1const date = new Date(2024, 0, 31);
2
3// Add 2 months to Jan 31
4date.setMonth(date.getMonth() + 2);
5console.log(date.toISOString());  // "2024-03-31T00:00:00.000Z" ✓
6
7// Add 1 month to Jan 31 (February doesn't have 31!)
8date.setMonth(date.getMonth() + 1);
9console.log(date.toISOString());  // "2024-03-02T00:00:00.000Z" ✓
10
11// JavaScript auto-adjusts overflow

Benefit: JavaScript automatically handles date overflow (Feb 31 → Mar 2 or 3 depending on leap year).

Best Practices

Performance

Avoid Frequent Date Creation

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JAVASCRIPT
1// BAD: Create new Date in loop
2for (let i = 0; i < 1000; i++) {
3  const now = new Date();
4  // ... operations
5}
6
7// GOOD: Create once, reuse
8const now = new Date();
9const timestamp = now.getTime();
10for (let i = 0; i < 1000; i++) {
11  // ... operations with timestamp
12}

Use Timestamp for Comparisons

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JAVASCRIPT
1const date1 = new Date(2024, 0, 8);
2const date2 = new Date(2024, 0, 15);
3
4// BAD: Compare Date objects (slow)
5if (date1 < date2) { }
6
7// GOOD: Compare timestamps (fast)
8if (date1.getTime() < date2.getTime()) { }
9
10// Even better: Use timestamps directly
11const ts1 = date1.getTime();
12const ts2 = date2.getTime();
13if (ts1 < ts2) { }

Cache Timezone Offset

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JAVASCRIPT
1// Cache timezone offset (doesn't change during runtime)
2const TZ_OFFSET = new Date().getTimezoneOffset() * 60 * 1000;
3
4// Use in conversions
5function toLocalTime(timestamp) {
6  return new Date(timestamp + TZ_OFFSET);
7}
8
9console.log(toLocalTime(Date.now()));

Data Integrity

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JAVASCRIPT
1// Validate before operations
2function safeDateOperation(callback) {
3  try {
4    const result = callback();
5    if (isNaN(result.getTime())) {
6      throw new Error('Invalid date result');
7    }
8    return result;
9  } catch (error) {
10    console.error('Date operation failed:', error);
11    return new Date();  // Return current time as fallback
12  }
13}
14
15// Usage
16const result = safeDateOperation(() => {
17  return new Date(invalidInput);
18});

Storage Best Practices

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JAVASCRIPT
1// Store as ISO 8601 string
2function serializeDate(date) {
3  return date.toISOString();
4}
5
6// Deserialize from ISO 8601
7function deserializeDate(isoString) {
8  return new Date(isoString);
9}
10
11// Usage in JSON
12const data = {
13  timestamp: Date.now(),
14  isoDate: new Date().toISOString(),
15  userCreated: serializeDate(new Date())
16};
17
18// Parse from API
19const createdDate = deserializeDate(apiResponse.userCreated);

Common Operations

Utility Functions

Start/End of Day

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JAVASCRIPT
1function startOfDay(date) {
2  const d = new Date(date);
3  d.setHours(0, 0, 0, 0);
4  return d;
5}
6
7function endOfDay(date) {
8  const d = new Date(date);
9  d.setHours(23, 59, 59, 999);
10  return d;
11}
12
13console.log(startOfDay(new Date()).toISOString());
14// "2024-01-08T00:00:00.000Z"

Start/End of Week

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JAVASCRIPT
1function startOfWeek(date) {
2  const d = new Date(date);
3  const day = d.getDay();
4  const diff = d.getDate() - day + (day === 0 ? -6 : 1);
5  d.setDate(diff);
6  return startOfDay(d);
7}
8
9function endOfWeek(date) {
10  const d = startOfWeek(new Date(date));
11  d.setDate(d.getDate() + 6);
12  return endOfDay(d);
13}
14
15console.log(startOfWeek(new Date(2024, 0, 8)).toISOString());
16// "2024-01-01T00:00:00.000Z" (Monday)

Start/End of Month

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JAVASCRIPT
1function startOfMonth(date) {
2  return new Date(date.getFullYear(), date.getMonth(), 1);
3}
4
5function endOfMonth(date) {
6  return new Date(date.getFullYear(), date.getMonth() + 1, 0);
7}
8
9console.log(startOfMonth(new Date(2024, 0, 15)).toISOString());
10// "2024-01-01T00:00:00.000Z"
11
12console.log(endOfMonth(new Date(2024, 0, 15)).toISOString());
13// "2024-01-31T00:00:00.000Z"

Age Calculation

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JAVASCRIPT
1function calculateAge(birthDate) {
2  const today = new Date();
3  const birth = new Date(birthDate);
4
5  let years = today.getFullYear() - birth.getFullYear();
6  const monthDiff = today.getMonth() - birth.getMonth();
7
8  if (monthDiff < 0 || (monthDiff === 0 && today.getDate() < birth.getDate())) {
9    years--;
10  }
11
12  return years;
13}
14
15console.log(calculateAge('1990-06-15'));
16// 33 (assuming current date is 2024-01-08)

Business Days Calculation

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JAVASCRIPT
1function addBusinessDays(startDate, days) {
2  const result = new Date(startDate);
3  let addedDays = 0;
4
5  while (addedDays < days) {
6    result.setDate(result.getDate() + 1);
7    const dayOfWeek = result.getDay();
8
9    if (dayOfWeek !== 0 && dayOfWeek !== 6) {  // Not weekend
10      addedDays++;
11    }
12  }
13
14  return result;
15}
16
17console.log(addBusinessDays(new Date(2024, 0, 8), 5).toISOString());
18// "2024-01-15T00:00:00.000Z" (skips weekend)

Examples and Use Cases

Countdown Timer

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JAVASCRIPT
1function countdown(targetDate) {
2  const now = Date.now();
3  const target = new Date(targetDate).getTime();
4  const diff = target - now;
5
6  if (diff <= 0) {
7    return { expired: true };
8  }
9
10  const days = Math.floor(diff / (1000 * 60 * 60 * 24));
11  const hours = Math.floor((diff % (1000 * 60 * 60 * 24)) / (1000 * 60 * 60));
12  const minutes = Math.floor((diff % (1000 * 60 * 60)) / (1000 * 60));
13  const seconds = Math.floor((diff % (1000 * 60)) / 1000);
14
15  return {
16    expired: false,
17    days,
18    hours,
19    minutes,
20    seconds,
21    totalMs: diff
22  };
23}
24
25console.log(countdown('2024-12-31'));
26// { expired: false, days: 357, hours: 11, minutes: 29, seconds: 12, totalMs: ... }

Relative Time (e.g., "2 hours ago")

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JAVASCRIPT
1function timeAgo(timestamp) {
2  const now = Date.now();
3  const diff = now - timestamp;
4  const seconds = Math.floor(diff / 1000);
5
6  if (seconds < 60) return 'just now';
7  const minutes = Math.floor(seconds / 60);
8  if (minutes < 60) return `${minutes} minute${minutes !== 1 ? 's' : ''} ago`;
9  const hours = Math.floor(minutes / 60);
10  if (hours < 24) return `${hours} hour${hours !== 1 ? 's' : ''} ago`;
11  const days = Math.floor(hours / 24);
12  if (days < 30) return `${days} day${days !== 1 ? 's' : ''} ago`;
13  const months = Math.floor(days / 30);
14  if (months < 12) return `${months} month${months !== 1 ? 's' : ''} ago`;
15  const years = Math.floor(days / 365);
16  return `${years} year${years !== 1 ? 's' : ''} ago`;
17}
18
19const oneHourAgo = Date.now() - (60 * 60 * 1000);
20console.log(timeAgo(oneHourAgo));  // "1 hour ago"

Date Range Validation

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JAVASCRIPT
1function isDateInRange(date, startDate, endDate) {
2  const timestamp = date.getTime();
3  return timestamp >= startDate.getTime() && timestamp <= endDate.getTime();
4}
5
6// Example: Check if date is in Q1 2024
7const q1Start = new Date(2024, 0, 1);
8const q1End = new Date(2024, 2, 31);
9const testDate = new Date(2024, 1, 15);
10
11console.log(isDateInRange(testDate, q1Start, q1End));  // true
12console.log(isDateInRange(new Date(2024, 3, 1), q1Start, q1End));  // false

Best Practices Summary

JavaScript Date Checklist

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TEXT
1✅ Always use ISO 8601 format ('2024-01-08T14:30:00.000Z') for strings
2✅ Work with timestamps internally, convert to Date only for display
3✅ Use toISOString() for API calls and database storage
4✅ Remember months are 0-indexed (0 = January, 11 = December)
5✅ Cache timezone offset and Date.now() to avoid repeated calls
6✅ Validate dates with isNaN(date.getTime())
7✅ Use Intl.DateTimeFormat for i18n-aware formatting
8✅ Consider libraries (date-fns, luxon, day.js) for complex operations
9✅ Use timestamp arithmetic for precision (add/subtract milliseconds)
10✅ Handle date overflow gracefully (JavaScript auto-adjusts)
11❌ Don't use locale-specific strings for parsing (inconsistent across browsers)
12❌ Don't mix UTC and local methods in same calculation
13❌ Don't create Date objects in loops (reuse or use timestamps)
14❌ Don't assume browser timezones (always validate with getTimezoneOffset)
15❌ Don't rely on automatic string parsing (use ISO 8601)
16❌ Don't forget about DST transitions when calculating business days

Performance Tips

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TEXT
1✅ Cache Date.now() instead of calling repeatedly
2✅ Use timestamps for comparisons (faster than Date objects)
3✅ Minimize Date object creation in loops
4✅ Use Math.floor() instead of bitwise operators for readability
5✅ Store timezone offset as constant for repeated calculations
6❌ Don't use Date.parse() on non-ISO strings (unreliable)
7❌ Don't create Date objects in hot code paths
8❌ Don't use toLocaleString() in performance-critical sections

Related Tools

• Unix Timestamp Converter - Convert Unix timestamps to dates
• Current Timestamp - Get current timestamp in multiple formats
• Timestamp Format Converter - Convert between timestamp formats
• JavaScript Timestamp Helper - JavaScript-specific examples
• UTC/Local Converter - Convert between UTC and local time

Additional Resources

• MDN: Date Object↗
• JavaScript Date Guide↗
• Intl.DateTimeFormat↗
• Time Zone Database↗

For production applications with complex date/time requirements, consider using libraries like:

• date-fns: Modular, immutable date utilities
• luxon: Modern API with timezone support (from Moment.js creators)
• day.js: Lightweight alternative to Moment.js (2KB)
• temporal-polyfill: Future TC39 Temporal API proposal

Native Date is sufficient for basic operations, but libraries provide better ergonomics and timezone handling for complex use cases.