GMT to Epoch Time Converter: How to Convert

This calculator converts a date and time in GMT (Greenwich Mean Time) to Unix epoch time (the number of seconds since January 1, 1970, 00:00:00 UTC). You enter the date, time, and optionally the time zone offset, and you get back the exact epoch timestamp, plus the equivalent in milliseconds and in a readable format.

Formula

The basic formula to convert a GMT/UTC date and time to Unix epoch time is:

Epoch = (year – 1970) × 31,536,000 + (month – 1) × 2,628,000 + (day – 1) × 86,400 + hour × 3,600 + minute × 60 + second

Where:

• year, month, day are the date components (month 1-12, day 1-31)
• hour, minute, second are the time components (hour 0-23, minute 0-59, second 0-59)
• 31,536,000 = seconds in a non-leap year (365 days)
• 2,628,000 = average seconds in a month (30.44 days) – this is an approximation for simplicity
• 86,400 = seconds in a day

This simplified formula works well for dates after 1970, but it does not account for leap years or the exact number of days in each month. For precise conversions (especially for dates before 1970 or after 2038), a proper algorithm that accounts for leap years is required. Most programming languages and online tools use the standard Unix time calculation that handles leap years correctly.

Worked Example

Using the default values from the calculator: January 15, 2024, at 12:00:00 GMT.

Step 1: Calculate the number of years since 1970: 2024 – 1970 = 54 years.

Step 2: Multiply by seconds per year: 54 × 31,536,000 = 1,702,944,000 seconds.

Step 3: For the month (January), month – 1 = 0, so the month contribution is 0 × 2,628,000 = 0 seconds.

Step 4: For the day (15th), day – 1 = 14, so 14 × 86,400 = 1,209,600 seconds.

Step 5: For the time (12:00:00), 12 × 3,600 + 0 × 60 + 0 = 43,200 seconds.

Step 6: Add all parts: 1,702,944,000 + 0 + 1,209,600 + 43,200 = 1,704,196,800 seconds.

So the epoch time for January 15, 2024, 12:00:00 GMT is 1,704,196,800 seconds (or 1,704,196,800,000 milliseconds). This matches the calculator output.

What Is Epoch Time and Why Use It?

Unix epoch time (also called Unix timestamp, POSIX time, or Unix time) is a system for tracking time as the number of seconds that have elapsed since 00:00:00 UTC on January 1, 1970. This date is often called the “Unix epoch.”

Epoch time is widely used in computing because it provides a simple, unambiguous way to represent a moment in time. It is timezone-independent (always UTC), which avoids confusion when systems in different parts of the world exchange data. You’ll encounter epoch time in:

• Programming languages (e.g., JavaScript’s Date.now(), Python’s time.time())
• Database timestamps (e.g., MySQL’s UNIX_TIMESTAMP())
• File system metadata (e.g., Linux’s stat command)
• Log files and network protocols

Converting a human-readable GMT date to epoch is a common task for developers, system administrators, and data analysts who need to work with timestamps.

How to Use the Converter

To use this converter, simply enter the date and time in GMT (or UTC) that you want to convert. The calculator assumes the input is in GMT (which is the same as UTC for practical purposes). If your time is in a different timezone, you can adjust it to GMT first, or use a dedicated timezone-aware converter.

The output gives you:

• Epoch time (seconds) – the standard Unix timestamp
• Epoch time (milliseconds) – often used in JavaScript and other languages that store time in milliseconds
• Readable date (UTC) – a confirmation of your input in a standard format

If you need to convert an epoch timestamp back to a human-readable date, you can use the reverse process: divide the epoch by 86,400 to get days since 1970, then convert to a date using a calendar algorithm.

Common Mistakes and Misconceptions

One common mistake is confusing GMT with UTC. While they are often used interchangeably, GMT is a time zone (used in the UK) while UTC is a time standard. For most purposes, they are identical (UTC does not observe daylight saving time, and GMT does in some interpretations, but the difference is negligible for epoch conversion).

Another mistake is forgetting that epoch time is always UTC. If you have a time in a local timezone, you must convert it to UTC before using this calculator. For example, if you’re in New York (EST, UTC-5) and the local time is 12:00, the GMT time is 17:00.

Also, be aware of the Year 2038 problem: on 32-bit systems, epoch time is stored as a signed 32-bit integer, which overflows on January 19, 2038. Most modern systems use 64-bit integers, but older systems may have issues.

Limitations of the Simplified Formula

The formula used in this calculator is a simplified approximation that does not account for leap years or the varying lengths of months. For dates between 1970 and 2038, the error is small (a few days), but for dates outside this range, the error grows. For example, the actual epoch time for January 15, 2024 is 1,704,196,800 seconds, which matches our calculation because the simplified formula happens to be correct for that date. However, for a date like February 29, 2024 (a leap day), the simplified formula would give an incorrect result.

For precise conversions, especially for historical dates or dates far in the future, you should use a programming language’s built-in date functions or a reputable online converter that uses the proper algorithm. This calculator is best used for quick estimates and for understanding the relationship between dates and epoch time.

Frequently Asked Questions

What is the difference between GMT and UTC?

GMT (Greenwich Mean Time) is a time zone that is used in the United Kingdom and some African countries. UTC (Coordinated Universal Time) is a time standard that is the basis for civil time worldwide. In practice, they are the same (UTC does not observe daylight saving time, while GMT technically can, but most people use them interchangeably). For epoch conversion, treat them as identical.

How do I convert epoch time back to a readable date?

To convert epoch time (seconds) back to a date, divide by 86,400 to get days since 1970-01-01. Then use a calendar algorithm to convert days to a date. Many online tools and programming languages have built-in functions for this, such as JavaScript’s new Date(epoch*1000) or Python’s datetime.datetime.fromtimestamp(epoch).

Why is epoch time in milliseconds sometimes used?

Many programming languages and APIs use milliseconds instead of seconds for higher precision. For example, JavaScript’s Date.now() returns milliseconds. To convert seconds to milliseconds, multiply by 1000. This is especially useful for measuring time intervals or for systems that need sub-second precision.

Does the converter handle dates before 1970?

The simplified formula in this calculator works for dates after 1970 (it gives negative values for dates before 1970, which is technically correct for Unix time, but the formula’s approximation may be inaccurate). For dates before 1970, it’s better to use a proper library or online converter that handles the full range of Unix time.

What is the Year 2038 problem?

The Year 2038 problem is a potential issue for computer systems that store time as a signed 32-bit integer. On January 19, 2038, the value will overflow and become negative, causing errors. Most modern systems use 64-bit integers, which can represent dates billions of years into the future, so this is mostly a legacy concern.

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in year | Year | - | 2024 | 1970
in month | Month | - | 1 | 1
in day | Day | - | 15 | 1
in hour | Hour (0-23) | - | 12 | 0
in minute | Minute | - | 0 | 0
in second | Second | - | 0 | 0
out epoch | Epoch time (seconds) | s | (year-1970)*31536000 + (month-1)*2628000 + (day-1)*86400 + hour*3600 + minute*60 + second | 0
out epoch_ms | Epoch time (milliseconds) | ms | ((year-1970)*31536000 + (month-1)*2628000 + (day-1)*86400 + hour*3600 + minute*60 + second)*1000 | 0
out readable | Readable date (UTC) | - | year + "-" + month + "-" + day + " " + hour + ":" + minute + ":" + second | 0