How to Format std::chrono Types Using fmtlib

fmtlib provides full formatting support for C++ chrono types through the fmt/chrono.h header, allowing you to format std::chrono::duration with specifiers like %Q and %q, and std::chrono::time_point using standard strftime-compatible patterns.

Formatting time values in C++ typically requires verbose stream manipulators or C-style format strings, but the fmtlib library eliminates this complexity with dedicated chrono support. By including the fmt/chrono.h header from the fmtlib/fmt repository, you gain access to type-safe formatting for durations, time points, and clocks. This guide demonstrates how to format std::chrono types using fmtlib with practical examples derived from the source code.

Include the fmt/chrono.h Header

All chrono formatting capabilities reside in a single header. Add the following include to your source file:

#include <fmt/chrono.h>

This header provides specializations of formatter<T> for std::chrono::duration and std::chrono::time_point types. When you pass chrono objects to fmt::format, the library automatically selects the appropriate formatter, parses the format string, and drives the output generation.

Format std::chrono::duration Values

Durations represent time intervals. The implementation in include/fmt/chrono.h provides the %Q specifier for the numeric value and %q for the unit suffix.

Basic Duration Formatting

Use {:%Q} to extract just the numeric value, or {:%Q %q} to include the unit:

#include <fmt/chrono.h>
#include <chrono>
#include <iostream>

int main() {
    using namespace std::chrono_literals;
    std::chrono::duration<double> d = 1.2345s;

    // Numeric value only
    std::cout << fmt::format("{:%Q}", d) << '\n';     // 1.2345
    
    // Value with unit
    std::cout << fmt::format("{:%Q %q}", d) << '\n';  // 1.2345 s
}

Precision and Fixed Formatting

For floating-point durations, you can specify precision within the format string. The library handles safe duration conversions internally through safe_duration_cast utilities (lines 200–260 in chrono.h) to prevent overflow:

// Fixed precision with 3 decimal places
std::cout << fmt::format("{:%Q.3f}", d) << '\n';  // 1.235

Format std::chrono::time_point Objects

Time points represent specific moments in time. The formatting implementation uses strftime-compatible specifiers processed by the tm_writer class (starting at line 440 in chrono.h).

System Clock Time Points

Format std::chrono::system_clock::now() using standard date and time specifiers:

#include <fmt/chrono.h>
#include <chrono>
#include <iostream>

int main() {
    using namespace std::chrono;
    auto now = system_clock::now();

    // ISO-8601 format
    std::cout << fmt::format("{:%F %T}", now) << '\n';  
    // Output: 2026-09-10 14:23:07
    
    // Custom format with weekday and month name
    std::cout << fmt::format("{:%a, %b %d %Y %H:%M}", now) << '\n';
    // Output: Sat, Sep 10 2026 14:23
}

UTC and Local Time with Subseconds

The library supports utc_clock and local time variants. For subsecond precision, the implementation uses write_fractional_seconds and write_floating_seconds (lines 660–720 in chrono.h):

#include <fmt/chrono.h>
#include <chrono>
#include <iostream>

int main() {
    using namespace std::chrono;
    utc_time<microseconds> utc_tp = utc_clock::now();

    // Include microseconds
    std::cout << fmt::format("{:%F %T.%Q}", utc_tp) << '\n';
    // Output: 2026-09-10 14:23:07.123456
}

Implementation Overview

The chrono formatting system in include/fmt/chrono.h relies on several key components to ensure safety and performance:

• parse_chrono_format (lines 28–73): Parses %-based format strings and delegates conversions to handler callbacks.
• tm_writer (line ~440): Receives callbacks like on_year, on_month, and on_second to write formatted characters into the output buffer.
• safe_duration_cast (lines 200–260): Ensures conversions between different duration types never overflow or lose precision during formatting.

These components work together within the generic formatter<T> specializations to provide zero-overhead, type-safe formatting.

Summary

• Include fmt/chrono.h to enable chrono formatting support in fmtlib.
• Use %Q for duration values and %q for unit suffixes when formatting std::chrono::duration.
• Apply standard strftime specifiers (like %Y, %m, %d, %H, %M, %S) to std::chrono::time_point objects.
• Access subsecond precision through fractional second specifiers supported by the internal write_fractional_seconds implementation.
• The formatting system prevents overflow through safe_duration_cast utilities during type conversions.

Frequently Asked Questions

What header do I need to include to format chrono types with fmtlib?

You must include <fmt/chrono.h>. This header provides the necessary formatter specializations for all std::chrono types including durations and time points. The rest of the fmtlib infrastructure is pulled in automatically when you include this file.

Can I format std::chrono::duration with custom precision?

Yes. For floating-point durations, you can specify precision directly in the format string using syntax like {:%Q.3f}. The library uses internal utilities like write_floating_seconds (located around lines 660–720 in include/fmt/chrono.h) to handle the fractional part formatting safely.

Does fmtlib support UTC and local time points?

Yes. The library provides full support for std::chrono::utc_time, std::chrono::local_time, and std::chrono::system_clock::time_point. All time point types use the same strftime-compatible formatting specifiers processed by the tm_writer class in chrono.h.

How does fmtlib prevent overflow when formatting durations?

The implementation uses safe_duration_cast utilities (located around lines 200–260 in include/fmt/chrono.h) to ensure conversions between different duration types maintain precision and avoid overflow during the formatting process.

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