Learn About Time

What is Unix Time?

Unix time, also known as POSIX time or epoch time, counts seconds since January 1, 1970 at 00:00:00 UTC (Coordinated Universal Time). This date is called the "Unix epoch."

It's widely used in computing for timestamps because it's a simple, continuously increasing number that's easy to compare, sort, and perform arithmetic operations on.

Example: Unix timestamp 1690646400 represents July 29, 2023 at 12:00:00 UTC.

The 2038 Problem

Systems using 32-bit signed integers to store Unix time will overflow on January 19, 2038 at 03:14:07 UTC, when the value exceeds 2^31−1 (2,147,483,647 seconds).

After this point, 32-bit systems will wrap around to negative numbers, potentially causing dates to appear as December 13, 1901. Modern 64-bit systems can handle dates until the year 292 billion!

Critical Date: January 19, 2038 at 03:14:07 UTC (2,147,483,647 seconds since epoch)

Daylight Saving Time (DST)

Daylight saving time shifts clocks forward by one hour during warmer months in many regions to extend evening daylight. This practice can cause complications in software:

Spring forward: 2:00 AM becomes 3:00 AM (one hour is skipped)
Fall back: 2:00 AM happens twice (one hour is repeated)
Different regions change on different dates

Always use timezone-aware libraries and store times in UTC when possible to avoid DST issues.

Working with Timezones

Timezones are regions with the same standard time. The world is divided into approximately 24 time zones, each typically 15 degrees of longitude apart.

JavaScript APIs

Intl.DateTimeFormat - Format dates in different timezones
Date.prototype.toLocaleString() - Locale-specific formatting
Temporal (upcoming) - Modern date/time API

Popular Libraries

date-fns-tz - Lightweight timezone support
luxon - Modern alternative to Moment.js
dayjs - Fast 2kB alternative

ISO 8601 Standard

ISO 8601 is the international standard for date and time representation. It provides unambiguous date/time formats that are machine-readable and sortable.

Common Formats:

Date: 2025-07-29
Date and Time: 2025-07-29T14:30:00
With Timezone: 2025-07-29T14:30:00Z (UTC)
With Offset: 2025-07-29T14:30:00+02:00

Time Precision & Accuracy

Precision Levels

Seconds: Standard Unix time
Milliseconds: JavaScript Date (1/1000 second)
Microseconds: High-precision systems (1/1,000,000 second)
Nanoseconds: Scientific applications (1/1,000,000,000 second)

Common Sources of Error

Network latency in distributed systems
Clock drift between machines
Leap seconds (rare UTC adjustments)
Timezone database updates

Best Practices for Developers

✅ Do This

Store timestamps in UTC in your database
Use timezone-aware libraries for calculations
Validate and sanitize time inputs
Consider leap years and DST transitions
Use ISO 8601 format for data exchange

❌ Avoid This

Storing local times without timezone info
Hardcoding timezone offsets
Ignoring DST transitions in calculations
Using string manipulation for date math
Assuming all days have 24 hours