MySQL TIMESTAMP vs. DATETIME: The Ultimate Showdown on Time Zones, UTC, and Storage

## Problem Background In daily database operations, developers often encounter questions about date and time types. For example, why can a `TIMESTAMP` column be directly compared with a string in the `'YYYY-MM-DD HH:MM:SS'` format? How does MySQL handle time zones for `TIMESTAMP`? And what are the underlying differences between it and `DATETIME`? This article, brought to you by the wiki.lib00.com team, will delve into these questions to help you fully understand the differences and best practices for these two core time types in MySQL. --- ## 1. The Magic Query: Why Can `TIMESTAMP` Be Compared with a String? A common query scenario looks like this: ```sql SELECT * FROM summary_hourly_lib00 WHERE hour_start >= '2025-01-01 12:00:00'; ``` Even though `hour_start` is a `TIMESTAMP`, this query works perfectly. The core reason behind this is **MySQL's Implicit Type Casting**. - **Intelligent Conversion**: When MySQL sees a `TIMESTAMP` or `DATETIME` column being compared with a well-formatted date-time string, its query optimizer is "smart" enough to recognize the string's intent. - **Conversion Process**: MySQL will convert the string on the right, `'2025-01-01 12:00:00'`, into an internal date-time value that matches the type of the column on the left. This conversion is performed based on the **current database connection's Session Time Zone**. - **Comparison Logic**: MySQL assumes the string represents a time in the current session's time zone. To compare it with the UTC timestamp stored in the `TIMESTAMP` column, it first converts this session time to UTC and then performs the comparison. **Conclusion**: Therefore, this syntax is not only feasible but also the standard and recommended practice by MySQL, as it keeps SQL statements highly readable. --- ## 2. The Core of `TIMESTAMP`: How Time Zones Are Handled The most fundamental difference between `TIMESTAMP` and `DATETIME` lies in its time zone handling mechanism, which can be summarized as: **Store in UTC, Retrieve in Session Time Zone**. ### Writing Data: Automatic Conversion to UTC MySQL doesn't care about the operating system time zone of your application server (e.g., PHP). It only cares about one thing: the **`time_zone` setting of the current database connection**. **The process is as follows**: 1. An application (e.g., a PHP script located at `/var/www/wiki.lib00.com/app/`) generates a time string, such as `'2025-01-01 12:00:00'`. 2. When this string is written to a `TIMESTAMP` column via an `INSERT` or `UPDATE` statement, the MySQL server receives it. 3. MySQL checks the `time_zone` setting of the current connection and assumes the incoming string is in that time zone. 4. Next, MySQL converts this time from the connection's time zone **to UTC (Coordinated Universal Time)**. 5. Finally, it stores the **Unix timestamp** (the number of seconds since 1970-01-01 00:00:00 UTC) corresponding to that UTC time in the database. ### Reading Data: Automatic Conversion Back to Session Time Zone When you `SELECT` a `TIMESTAMP` column, MySQL performs the reverse operation. **The process is as follows**: 1. MySQL reads the stored UTC timestamp from the table. 2. It again checks the **`time_zone` setting of the current database connection**. 3. It converts the UTC timestamp **back to the time corresponding to that connection's time zone**. 4. Finally, it returns the result as a string in the `'YYYY-MM-DD HH:MM:SS'` format to the client. ### Practical Example - **Connection A (Beijing, UTC+8)**: Executes `SET time_zone = '+08:00';` after connecting. - **Connection B (New York, UTC-5)**: Executes `SET time_zone = '-05:00';` after connecting. **Operations**: 1. **Beijing Connection Writes Data**: ```sql INSERT INTO my_events_lib00 (event_time) VALUES ('2025-01-01 12:00:00'); ``` MySQL treats this as 12:00 PM Beijing time, converts it to `2025-01-01 04:00:00` UTC, and stores the corresponding timestamp. 2. **New York Connection Reads Data**: ```sql SELECT event_time FROM my_events_lib00; ``` MySQL reads the UTC timestamp, converts it to the New York time zone (UTC-5), and the returned result will be `'2024-12-31 23:00:00'`. --- ## 3. `TIMESTAMP` vs. `DATETIME`: An Architect's Guide to Choosing `TIMESTAMP` is fundamentally a UTC timestamp, while `DATETIME` is completely different. Understanding their differences is key to making the right technical choices. | Feature | `TIMESTAMP` | `DATETIME` | | :--- | :--- | :--- | | **Storage** | **UTC Timestamp** (4-byte integer) | **"What you see is what you get"** (Composite YYYYMMDDHHMMSS value, 8 bytes) | | **Time Zone Handling** | **Timezone-aware** (Automatic conversion) | **Timezone-naive** (Stores and retrieves literally) | | **Range** | `1970-01-01 00:00:01` UTC to `2038-01-19 03:14:07` UTC | `1000-01-01 00:00:00` to `9999-12-31 23:59:59` | | **Storage Space** | Fixed 4 bytes | Typically 8 bytes (5 bytes + fractional seconds) | ### When to Use `TIMESTAMP`? Based on DP's experience, `TIMESTAMP` is best suited for recording a **globally unique, absolute point in time**. - **Scenarios**: `created_at`, `updated_at`, `last_login_time`, etc. - **Advantages**: Ideal for applications that require internationalization. Users in any time zone will automatically see the correct local time without any extra conversion logic in the application layer. ### When to Use `DATETIME`? `DATETIME` is used to record a **literal time** that is independent of any time zone. - **Scenarios**: A user's birthday, a future meeting time at a specific location (e.g., "May 10, 2025, 9:00 AM at the New York office"). The meaning of these times should not change with the time zone. - **Advantages**: Has a much wider range and its behavior is intuitive, as it is not affected by the session's time zone. --- ## Summary - **`TIMESTAMP`** is like a **world clock**. It records an absolute point in time, and its displayed value (local time) varies depending on the observer's time zone. - **`DATETIME`** is like a **calendar clock on the wall**. It records a literal time, and its value remains fixed no matter where you view it from. Correctly understanding and using `TIMESTAMP` versus `DATETIME` is fundamental to designing robust and scalable systems. We hope this in-depth analysis from wiki.lib00 helps you make wiser decisions in your future development work.