---
title: 5.7 difftime()
description: "## Overview The `difftime()` function calculates the difference between two calendar times. The result is returned as a value of type `double` representing the elapsed time in **seconds**. `difftime()` is commonly used to determine the duration between two events. ##…"
url: "https://edcret.in/docs/linux-os/system-calls/time-system-calls/c-difftime-function/"
updated: "2026-07-27"
category: Linux OS / System Calls / Time System Calls
---

# 5.7 difftime()

## Overview

The `difftime()` function calculates the difference between two calendar times.

The result is returned as a value of type `double` representing the elapsed time in **seconds**.

`difftime()` is commonly used to determine the duration between two events.

## Header File

```
#include <time.h>
```

## Man Pages

```
man 3 difftime
```

## Function Prototype

```
double difftime(time_t end, time_t beginning);
```

## Parameters

| Parameter | Description |
| --- | --- |
| `end` | Ending calendar time. |
| `beginning` | Starting calendar time. |

## Return Value

Returns the difference between `end` and `beginning` in **seconds** as a value of type `double`.

The returned value can be:

- Positive if `end` is later than `beginning`.
- Zero if both times are equal.
- Negative if `end` is earlier than `beginning`.

## Examples

### Example 1 – Calculate Elapsed Time

#### main.c

```
#include <stdio.h>
#include <time.h>
#include <unistd.h>

int main(void)
{
    time_t start, end;
    double elapsed_time;

    start = time(NULL);

    sleep(5);

    end = time(NULL);

    elapsed_time = difftime(end, start);

    printf("Elapsed Time = %.0f seconds\n", elapsed_time);

    return 0;
}
```

#### Output

```
Elapsed Time = 5 seconds
```

> The output may vary slightly depending on your system.

#### Explanation

```
start = time(NULL);
```

Stores the starting calendar time.

```
sleep(5);
```

Pauses the program for five seconds.

```
end = time(NULL);
```

Stores the ending calendar time.

```
elapsed_time = difftime(end, start);
```

Calculates the difference between the two calendar times.

The returned value is expressed in seconds.

---

### Example 2 – Calculate the Difference Between Two Time Values

#### main.c

```
#include <stdio.h>
#include <time.h>

int main(void)
{
    time_t start = 100;
    time_t end = 250;

    printf("Difference = %.0f seconds\n",
           difftime(end, start));

    return 0;
}
```

#### Output

```
Difference = 150 seconds
```

#### Explanation

```
time_t start = 100;
time_t end = 250;
```

Creates two calendar time values.

```
difftime(end, start);
```

Returns the difference between the two values.

Since `250 - 100 = 150`, the function returns `150`.

## Notes

- `difftime()` returns the difference between two calendar times.
- The returned value is expressed in **seconds**.
- The return type is `double`.
- `end` should normally represent a later time than `beginning`.
- `difftime()` should be preferred over manually subtracting two `time_t` values because it provides a portable implementation regardless of how `time_t` is represented.

## Related Functions

- `time()`
- `clock()`
- `clock_gettime()`
- `ctime()`
- `localtime()`
- `gmtime()`