Epoch Time Explained

Introduction

Epoch time (also called Unix timestamp or POSIX time) is a system for describing a point in time as the number of seconds that have elapsed since a specific reference date and time. Understanding epoch time is fundamental for developers, system administrators, and anyone working with timestamps in computing.

What is Epoch Time

Definition

Epoch time represents time as a continuous count of time units (usually seconds) from a fixed reference point called the "epoch":

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1Epoch Timestamp = Current Time - Epoch Start Time
2
3Example (Unix Epoch):
4  Epoch Start: January 1, 1970, 00:00:00 UTC
5  Current Time: January 8, 2024, 00:00:00 UTC
6  Timestamp: 1704672000 seconds since epoch

Unix Epoch (POSIX Time)

The most widely used epoch system is Unix epoch (POSIX time):

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1Unix Epoch Reference:
2  Start Date: January 1, 1970, 00:00:00 UTC
3  Time Units: Seconds (some systems use milliseconds)
4  Notation: 32-bit signed integer (common) or 64-bit
5  Max Range (32-bit): 1901-12-13 to 2038-01-19
6  Current Use: Linux, Unix, macOS, most programming languages

Interesting Fact: Unix epoch was chosen because it was a round number at the time Unix was being developed, making calculations simple and avoiding negative timestamps for most practical uses.

History of Unix Epoch

Unix epoch was established in the early 1970s:

• 1970: Unix epoch date chosen (January 1, 1970)
• 1971-1973: Initial Unix systems used time_t type (32-bit)
• 1988: POSIX standard formally adopted Unix time
• 2000s: Most systems transitioned to 64-bit time_t to avoid year 2038 problem
• 2020s: Modern systems use 64-bit epoch timestamps

Why 1970? It was approximately the time Unix was being developed and represents a convenient "zero point" before most computer systems existed, avoiding negative timestamps for early computing history.

Different Epoch Systems

Common Epoch References

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11. Unix Epoch (POSIX)
2   Start: January 1, 1970, 00:00:00 UTC
3   Units: Seconds (or milliseconds)
4   Usage: Unix, Linux, macOS, most programming languages
5
62. Windows FILETIME
7   Start: January 1, 1601, 00:00:00 UTC
8   Units: 100-nanosecond intervals
9   Usage: Windows file systems, NTFS
10
113. Mac HFS/HFS+ Epoch
12   Start: January 1, 1904, 00:00:00 UTC
13   Units: Seconds
14   Usage: Classic Mac OS (pre-OS X)
15
164. GPS Epoch
17   Start: January 6, 1980, 00:00:00 UTC
18   Units: Seconds
19   Usage: GPS systems, navigation
20
215. NTP Timestamp
22   Start: January 1, 1900, 00:00:00 UTC
23   Units: Seconds (64-bit)
24   Usage: Network Time Protocol
25
266. WebKit/Chrome Epoch
27   Start: January 1, 1601, 00:00:00 UTC
28   Units: Microseconds
29   Usage: Web browsers (Date object)

Important: Always verify which epoch system your system or API uses before converting timestamps.

JavaScript Epoch

JavaScript uses Unix epoch in milliseconds:

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JAVASCRIPT
1// JavaScript Date uses Unix epoch in milliseconds
2const now = Date.now();
3console.log(now);  // e.g., 1704672000000 (milliseconds)
4
5// Convert to seconds
6const nowSec = Math.floor(Date.now() / 1000);
7console.log(nowSec);  // e.g., 1704672000 (seconds)
8
9// Create from Unix epoch seconds
10const date = new Date(1704672000 * 1000);
11console.log(date.toISOString());  // "2024-01-08T00:00:00.000Z"

Python Epoch

Python's time.time() uses Unix epoch in seconds:

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PYTHON
1import time
2from datetime import datetime
3
4# Get current Unix timestamp (seconds)
5now_ts = time.time()
6print(now_ts)  # e.g., 1704672000.123
7
8# Create datetime from Unix timestamp
9dt = datetime.fromtimestamp(1704672000)
10print(dt)  # 2024-01-08 00:00:00
11
12# Get UTC timestamp
13now_utc = datetime.now(timezone.utc).timestamp()
14print(now_utc)  # e.g., 1704672000

Java Epoch

Java uses Unix epoch in milliseconds (consistent with JavaScript):

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JAVA
1import java.time.Instant;
2
3// Get current timestamp (milliseconds)
4long nowMs = System.currentTimeMillis();
5System.out.println(nowMs);  // e.g., 1704672000000
6
7// Get from Instant (Java 8+)
8Instant now = Instant.now();
9long ts = now.getEpochSecond();
10System.out.println(ts);  // e.g., 1704672000
11
12// Create Instant from Unix timestamp
13Instant instant = Instant.ofEpochSecond(1704672000);
14System.out.println(instant);  // 2024-01-08T00:00:00Z

Timestamp Limits and Ranges

32-bit Signed Integer Limits

Traditional Unix timestamps use 32-bit signed integers:

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132-bit Signed Integer Range:
2  Min Value: -2147483648 (seconds)
3  Max Value: 2147483647 (seconds)
4  Min Date: December 13, 1901, 20:45:52 UTC
5  Max Date: January 19, 2038, 03:14:07 UTC
6  Current Date: January 8, 2024
7  Time to Overflow: ~14 years

Critical: January 19, 2038, 03:14:07 UTC is known as the "Year 2038 problem" - 32-bit Unix timestamps will overflow and wrap to negative values, breaking many systems.

64-bit Timestamps

Modern systems use 64-bit timestamps to solve overflow issues:

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164-bit Signed Integer Range:
2  Min Value: -9223372036854775808 (seconds)
3  Max Value: 9223372036854775807 (seconds)
4  Min Date: ~293 billion years BCE
5  Max Date: ~293 billion years CE
6  Practical Limit: Effectively infinite for all practical purposes
7  Current Date: January 8, 2024

Best Practice: Always use 64-bit timestamps in new systems to avoid year 2038 problem and other overflow issues.

Millisecond vs Second Precision

Different systems use different precision:

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1Second Precision (Traditional Unix):
2  Units: Seconds since epoch
3  Range (32-bit): 136 years
4  Use: Unix/Linux systems, APIs, databases
5  Example: 1704672000
6
7Millisecond Precision (Modern Systems):
8  Units: Milliseconds since epoch
9  Range (32-bit): ~49 days (not practical)
10  Range (64-bit): ~293 million years
11  Use: JavaScript, Java, most modern APIs
12  Example: 1704672000000

Conversion Formula:

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1Seconds to Milliseconds: seconds * 1000
2Milliseconds to Seconds: milliseconds / 1000

The Year 2038 Problem

What is the Year 2038 Problem?

The Year 2038 problem is a time computing bug where 32-bit signed Unix timestamps will overflow on January 19, 2038, at 03:14:07 UTC.

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1Overflow Timestamp: 2147483647
2Date: January 19, 2038, 03:14:07 UTC
3
4After Overflow (Signed 32-bit wraps to negative):
5Next Second: -2147483648
6Date: December 13, 1901, 20:45:52 UTC

Critical Impact: Systems using 32-bit timestamps will fail or produce incorrect dates after 2038-01-19.

Affected Systems

Systems potentially affected by year 2038 problem:

• Legacy Unix systems still using 32-bit time_t
• Embedded systems with limited memory
• Older database systems
• File systems using 32-bit timestamps
• Network protocols with 32-bit time fields
• Some APIs still using 32-bit integers

Solutions

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1Solution 1: Upgrade to 64-bit
2  - Use time64_t or equivalent
3  - Increases range to ~293 billion years
4  - Required for long-term systems
5
6Solution 2: Offset Timestamps
7  - Store timestamps with epoch offset
8  - Example: Store "years since 2000" instead of seconds since 1970
9  - Requires conversion on read/write
10
11Solution 3: Use Alternative Formats
12  - Use ISO 8601 strings (no overflow)
13  - Use datetime types with larger range
14  - Store year, month, day separately

Prevention: When designing new systems, always use 64-bit timestamps or datetime objects with larger ranges.

Converting Between Epochs

Unix to Windows FILETIME

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JAVASCRIPT
1function unixToWindowsFiletime(unixTimestamp) {
2  // Unix to FILETIME: 100-nanosecond intervals since 1601-01-01
3  const WINDOWS_EPOCH = 0x019DB1DED53E8000; // FILETIME of Unix epoch
4  const NANOSECONDS_PER_MILLISECOND = 10000;
5
6  const filetime = (unixTimestamp * 1000 + 11644473600000) * NANOSECONDS_PER_MILLISECOND;
7
8  // For large numbers, use BigInt
9  return BigInt(unixTimestamp) * 10000n + 116444736000000000n;
10}
11
12// Example
13const filetime = unixToWindowsFiletime(1704672000);
14console.log(filetime);  // 1336477752000000000

Windows FILETIME to Unix

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JAVASCRIPT
1function windowsFiletimeToUnix(filetime) {
2  const NANOSECONDS_PER_MILLISECOND = 10000;
3
4  // FILETIME to Unix: subtract Windows epoch
5  const unixMs = (filetime - 116444736000000000) / 10000n;
6
7  return Number(unixMs / 1000n);
8}
9
10// Example
11const filetime = 133647752000000000n;
12const unixTs = windowsFiletimeToUnix(filetime);
13console.log(unixTs);  // 1704672000

Unix to GPS Time

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PYTHON
1import datetime
2
3def unix_to_gps(unix_timestamp):
4    """Convert Unix timestamp to GPS timestamp"""
5    # GPS epoch: January 6, 1980
6    # Unix epoch: January 1, 1970
7    # Difference: 315964800 seconds
8    GPS_EPOCH_OFFSET = 315964800
9
10    return unix_timestamp - GPS_EPOCH_OFFSET
11
12def gps_to_unix(gps_timestamp):
13    """Convert GPS timestamp to Unix timestamp"""
14    GPS_EPOCH_OFFSET = 315964800
15    return gps_timestamp + GPS_EPOCH_OFFSET
16
17# Example
18unix_ts = 1704672000
19gps_ts = unix_to_gps(unix_ts)
20print(f"GPS Timestamp: {gps_ts}")  # 5355687200

Unix to Mac HFS Time

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PYTHON
1import datetime
2
3def unix_to_mac_hfs(unix_timestamp):
4    """Convert Unix timestamp to Mac HFS timestamp"""
5    # Mac HFS epoch: January 1, 1904
6    # Unix epoch: January 1, 1970
7    # Difference: -2082844800 seconds (Mac is earlier)
8    MAC_EPOCH_OFFSET = -2082844800
9
10    return unix_timestamp - MAC_EPOCH_OFFSET
11
12def mac_hfs_to_unix(mac_timestamp):
13    """Convert Mac HFS timestamp to Unix timestamp"""
14    MAC_EPOCH_OFFSET = -2082844800
15    return mac_timestamp + MAC_EPOCH_OFFSET
16
17# Example
18unix_ts = 1704672000
19mac_ts = unix_to_mac_hfs(unix_ts)
20print(f"Mac HFS Timestamp: {mac_ts}")  # 3787516800

Practical Applications

Calculating Duration

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PYTHON
1import time
2
3def calculate_duration(start_ts, end_ts):
4    """Calculate duration between two Unix timestamps"""
5    duration_seconds = end_ts - start_ts
6
7    hours = duration_seconds // 3600
8    minutes = (duration_seconds % 3600) // 60
9    seconds = duration_seconds % 60
10
11    return f"{hours}h {minutes}m {seconds}s"
12
13# Example
14start = 1704587200  # 2024-01-07 00:00:00
15end = 1704673600    # 2024-01-08 00:00:00
16print(calculate_duration(start, end))  # "24h 0m 0s"

Future/Past Time Calculation

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JAVASCRIPT
1function timeUntil(targetTimestamp) {
2  const now = Date.now() / 1000;
3  const diff = targetTimestamp - now;
4
5  if (diff <= 0) {
6    return { type: 'past', text: 'Already passed' };
7  }
8
9  const days = Math.floor(diff / 86400);
10  const hours = Math.floor((diff % 86400) / 3600);
11  const minutes = Math.floor((diff % 3600) / 60);
12
13  return {
14    type: 'future',
15    text: `${days} days, ${hours} hours, ${minutes} minutes`
16  };
17}
18
19// Calculate time until end of 2037
20const target = 2155999999; // December 31, 2037
21console.log(timeUntil(target));
22// "511 days, 2 hours, 33 minutes"

Date Range Validation

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PYTHON
1import time
2
3def is_valid_unix_timestamp(ts, allow_future=True):
4    """Validate Unix timestamp is within reasonable range"""
5    # Minimum: January 1, 1970
6    MIN_TIMESTAMP = 0
7
8    # Maximum: January 19, 2038 (32-bit limit)
9    MAX_TIMESTAMP = 2147483647
10
11    if ts < MIN_TIMESTAMP:
12        return False, "Timestamp is before Unix epoch"
13
14    if not allow_future and ts > time.time():
15        return False, "Timestamp is in the future"
16
17    if ts > MAX_TIMESTAMP:
18        return False, "Timestamp exceeds 32-bit limit"
19
20    return True, "Valid"
21
22# Example
23print(is_valid_unix_timestamp(1704672000))  # (True, "Valid")
24print(is_valid_unix_timestamp(-1000))  # (False, "Timestamp is before Unix epoch")
25print(is_valid_unix_timestamp(3000000000))  # (False, "Timestamp exceeds 32-bit limit")

Batch Conversion

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JAVASCRIPT
1function convertUnixTimestamps(timestamps) {
2  return timestamps.map(ts => {
3    const date = new Date(ts * 1000);
4    return {
5      timestamp: ts,
6      isoString: date.toISOString(),
7      utcString: date.toUTCString(),
8      localString: date.toLocaleString()
9    };
10  });
11}
12
13// Example batch conversion
14const timestamps = [1704587200, 1704673600, 1704760000];
15const results = convertUnixTimestamps(timestamps);
16console.log(results);
17/*
18[
19  {
20    timestamp: 1704587200,
21    isoString: "2024-01-07T00:00:00.000Z",
22    utcString: "Sun, 07 Jan 2024 00:00:00 GMT",
23    localString: "1/7/2024, 12:00:00 AM"
24  },
25  ...
26]
27*/

Best Practices

Timestamp Selection Guidelines

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1When choosing timestamp precision:
2  ✅ Use seconds for Unix/Linux compatibility
3  ✅ Use milliseconds for JavaScript/Java/Web compatibility
4  ✅ Use 64-bit integers for future-proof systems
5  ✅ Use ISO 8601 strings for cross-system compatibility
6  ✅ Document epoch system in API specifications
7  ❌ Don't mix precision levels in same API
8  ❌ Don't use 32-bit timestamps for long-term systems
9  ❌ Don't assume all systems use Unix epoch

Storage Recommendations

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1Database storage:
2  ✅ Store as TIMESTAMP or DATETIME (64-bit)
3  ✅ Store timezone information separately
4  ✅ Use UTC for all storage
5  ✅ Validate timestamp ranges on input
6  ❌ Don't store as VARCHAR or STRING
7  ❌ Don't store local times without timezone metadata
8  ❌ Don't use 32-bit integers for new systems

API Design

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1REST API design:
2  ✅ Use ISO 8601 in JSON responses (e.g., "2024-01-08T00:00:00Z")
3  ✅ Include timezone information (Z or offset)
4  ✅ Document timestamp precision (seconds or milliseconds)
5  ✅ Support both input formats for flexibility
6  ✅ Return descriptive errors for invalid timestamps
7  ❌ Don't use locale-specific formats in APIs
8  ❌ Don't assume client timezone
9  ❌ Don't silently correct invalid timestamps

Testing Strategies

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PYTHON
1import datetime
2
3def test_epoch_conversions():
4    """Test epoch conversion edge cases"""
5    test_cases = [
6        ("Unix epoch start", 0, datetime.datetime(1970, 1, 1)),
7        ("Current time", 1704672000, datetime.datetime(2024, 1, 8)),
8        ("Year 2038 limit", 2147483647, datetime.datetime(2038, 1, 19, 3, 14, 7)),
9        ("Negative timestamp", -86400, datetime.datetime(1969, 12, 31, 0, 0, 0)),
10    ]
11
12    for name, timestamp, expected in test_cases:
13        result = datetime.datetime.fromtimestamp(timestamp, tz=datetime.timezone.utc)
14        assert result == expected, f"Failed: {name}"
15
16    print("All epoch conversion tests passed!")
17
18test_epoch_conversions()

Related Tools

• Unix Timestamp Converter - Convert Unix timestamps to dates
• Timestamp Format Converter - Convert between multiple epoch formats
• Current Timestamp - Get current timestamp in multiple formats
• Timestamp Validator - Validate timestamp ranges and formats
• Excel Timestamp Converter - Convert Excel serial dates to Unix timestamps

Additional Resources

• POSIX Time Definition↗
• Unix Time FAQ↗
• Year 2038 Problem↗
• Epoch Converter↗
• NTP Timestamp Format↗

For most applications, Unix epoch (January 1, 1970) is the de facto standard. However, always verify the epoch system when integrating with external systems, APIs, or legacy data, as different systems may use different reference dates.

Quick Reference

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1Common Epoch Timestamps:
2  0: January 1, 1970, 00:00:00 UTC (Unix epoch start)
3  946684800: January 1, 2000, 00:00:00 UTC
4  1577836800: January 1, 2020, 00:00:00 UTC
5  1704067200: January 1, 2024, 00:00:00 UTC
6  2147483647: January 19, 2038, 03:14:07 UTC (32-bit max)
7  253402300799: December 31, 9999, 23:59:59 UTC (Common limit)