Unix タイムスタンプについて: 開発者ガイド |ケイティのブログ

If you've worked with databases, APIs, or any system that tracks time, you've encountered Unix timestamps. This simple yet powerful concept is fundamental to how computers handle time. Let's demystify it.

Unix タイムスタンプとは何ですか?

A Unix timestamp (also called "epoch time" or "POSIX time") is the number of seconds that have elapsed since January 1, 1970, 00:00:00 UTC. This moment is called the "Unix epoch."

Example: The timestamp 1705276800 represents January 15, 2024, 00:00:00 UTC. It's simply the count of seconds since the epoch.

Why 1970? When Unix was being developed in the early 1970s, developers needed a starting point. January 1, 1970 was chosen as a convenient, recent date that predated the system's creation.

開発者がタイムスタンプを好む理由

Language-agnostic: A timestamp means the same thing in JavaScript, Python, Java, or any language
Timezone-independent: Timestamps are always UTC, eliminating timezone confusion
Easy to compare: Comparing two timestamps is just comparing two numbers
Easy math: Add 86400 to get tomorrow. Subtract 3600 to go back an hour.
Compact storage: A 32-bit integer stores a date more efficiently than a string

タイムスタンプの操作

JavaScript

                // Current timestamp (in seconds)

                const timestamp = Math.floor(Date.now() / 1000);

                // Timestamp to Date

                const date = new Date(timestamp * 1000);

                // Date to timestamp

                const ts = Math.floor(new Date('2024-01-15').getTime() / 1000);

⚠️ JavaScript に関する注意点

JavaScript's Date.now() returns milliseconds, not seconds. Always divide by 1000 for Unix timestamps.

パイソン

                import time

                from datetime import datetime

                # Current timestamp

                timestamp = int(time.time())

                # Timestamp to datetime

                dt = datetime.fromtimestamp(timestamp)

                # Datetime to timestamp

                ts = int(datetime(2024, 1, 15).timestamp())

PHP

                // Current timestamp

                $timestamp = time();

                // Timestamp to date

                $date = date('Y-m-d H:i:s', $timestamp);

                // Date to timestamp

                $ts = strtotime('2024-01-15');

2038年問題

⚠️ Critical Issue: On January 19, 2038, at 03:14:07 UTC, 32-bit Unix timestamps will overflow. Systems still using 32-bit signed integers for time will see dates wrap around to 1901 or crash entirely.

The maximum value of a signed 32-bit integer is 2,147,483,647 seconds after the epoch, which equals January 19, 2038. After this moment, the counter overflows.

Modern systems address this by using 64-bit timestamps, which won't overflow for about 292 billion years. Most programming languages and databases have already transitioned.

よくある落とし穴

1. タイムゾーンの混乱

Timestamps are always UTC. When converting to local time, remember to apply the timezone offset:

                // Wrong: treats UTC timestamp as local time

                new Date(1705276800000) // may show wrong time

                // Right: explicitly handle timezone

                new Date(1705276800000).toLocaleString('en-US', {timeZone: 'America/New_York'})

2. ミリ秒と秒

JavaScript uses milliseconds. Unix traditionally uses seconds. Always check which your system expects:

Unix timestamp: 1705276800 (10 digits)
JavaScript timestamp: 1705276800000 (13 digits)

3. 負のタイムスタンプ

Dates before 1970 have negative timestamps. December 31, 1969 at 23:59:59 UTC is timestamp -1. Some systems don't handle negative timestamps correctly.

4.うるう秒

Unix time doesn't account for leap seconds. It assumes every day is exactly 86,400 seconds. For most applications this doesn't matter, but for high-precision time systems, consider NTP or TAI.

有用なタイムスタンプ値

                Seconds in a minute: 60

                Seconds in an hour: 3,600

                Seconds in a day: 86,400

                Seconds in a week: 604,800

                Seconds in a year: 31,536,000 (approximately)

                // Quick math

                tomorrow = now + 86400

                nextWeek = now + 604800

                yesterday = now - 86400

タイムスタンプと日付文字列を使用する場合

Use timestamps for:

Database storage (consistent, sortable, efficient)
API communication (no timezone ambiguity)
Log files (easy to compare)
Session expiration, cache invalidation

Use date strings (ISO 8601) for:

Human-readable logs
User-facing interfaces
When timezone information must be preserved

🔧 タイムスタンプを変換する必要がありますか?

Use our free Unix Timestamp Converter to convert between timestamps and human-readable dates.

タイムスタンプコンバータを開く →

結論

Unix timestamps are beautifully simple: count seconds from a fixed point. This simplicity makes them perfect for storing, comparing, and transmitting time data. Just remember the key conversions, watch out for the millisecond/second distinction, and always think in UTC.

With timestamps in your toolkit, handling time in any application becomes straightforward.

⏰ Unix タイムスタンプについて: 開発者ガイド