Implement locale-aware hour formatting for date command

.vscode/cspell.dictionaries/jargon.wordlist.txt

@@ -76,6 +76,7 @@ iflag
 iflags
 kibi
 kibibytes
+langinfo
 libacl
 lcase
 listxattr
@@ -129,6 +130,7 @@ semiprimes
 setcap
 setfacl
 setfattr
+setlocale
 shortcode
 shortcodes
 siginfo
@@ -163,6 +165,8 @@ xattrs
 xpass
 
 # * abbreviations
+AMPM
+ampm
 consts
 deps
 dev

src/uu/date/src/date.rs

@@ -5,6 +5,8 @@
 
 // spell-checker:ignore strtime ; (format) DATEFILE MMDDhhmm ; (vars) datetime datetimes getres AWST ACST AEST
 
+mod locale;
+
 use clap::{Arg, ArgAction, Command};
 use jiff::fmt::strtime;
 use jiff::tz::{TimeZone, TimeZoneDatabase};
@@ -537,7 +539,7 @@ fn make_format_string(settings: &Settings) -> &str {
         },
         Format::Resolution => "%s.%N",
         Format::Custom(ref fmt) => fmt,
-        Format::Default => "%a %b %e %X %Z %Y",
+        Format::Default => locale::get_locale_default_format(),
     }
 }
 

src/uu/date/src/locale.rs

@@ -0,0 +1,167 @@
+// This file is part of the uutils coreutils package.
+//
+// For the full copyright and license information, please view the LICENSE
+// file that was distributed with this source code.
+
+//! Locale detection for time format preferences
+
+// nl_langinfo is available on glibc (Linux), Apple platforms, and BSDs
+// but not on Android, Redox or other minimal Unix systems
+
+// Macro to reduce cfg duplication across the module
+macro_rules! cfg_langinfo {
+    ($($item:item)*) => {
+        $(
+            #[cfg(any(
+                target_os = "linux",
+                target_vendor = "apple",
+                target_os = "freebsd",
+                target_os = "netbsd",
+                target_os = "openbsd",
+                target_os = "dragonfly"
+            ))]
+            $item
+        )*
+    }
+}
+
+cfg_langinfo! {
+    use std::ffi::CStr;
+    use std::sync::OnceLock;
+}
+
+cfg_langinfo! {
+    /// Cached result of locale time format detection
+    static TIME_FORMAT_CACHE: OnceLock<bool> = OnceLock::new();
+
+    /// Internal function that performs the actual locale detection
+    fn detect_12_hour_format() -> bool {
+        unsafe {
+            // Set locale from environment variables (empty string = use LC_TIME/LANG env vars)
+            libc::setlocale(libc::LC_TIME, c"".as_ptr());
+
+        // Get the date/time format string from locale
+        let d_t_fmt_ptr = libc::nl_langinfo(libc::D_T_FMT);
+        if d_t_fmt_ptr.is_null() {
+            return false;
+        }
+
+        let Ok(format) = CStr::from_ptr(d_t_fmt_ptr).to_str() else {
+            return false;
+        };
+
+        // Check for 12-hour indicators first (higher priority)
+        // %I = hour (01-12), %l = hour (1-12) space-padded, %r = 12-hour time with AM/PM
+        if format.contains("%I") || format.contains("%l") || format.contains("%r") {
+            return true;
+        }
+
+        // If we find 24-hour indicators, it's definitely not 12-hour
+        // %H = hour (00-23), %k = hour (0-23) space-padded, %R = %H:%M, %T = %H:%M:%S
+        if format.contains("%H")
+            || format.contains("%k")
+            || format.contains("%R")
+            || format.contains("%T")
+        {
+            return false;
+        }
+
+        // Also check the time-only format as a fallback
+        let t_fmt_ptr = libc::nl_langinfo(libc::T_FMT);
+        let mut time_fmt_opt = None;
+        if !t_fmt_ptr.is_null() {
+            if let Ok(time_format) = CStr::from_ptr(t_fmt_ptr).to_str() {
+                time_fmt_opt = Some(time_format);
+                if time_format.contains("%I")
+                    || time_format.contains("%l")
+                    || time_format.contains("%r")
+                {
+                    return true;
+                }
+            }
+        }
+
+        // Check if there's a specific 12-hour format defined
+        let t_fmt_ampm_ptr = libc::nl_langinfo(libc::T_FMT_AMPM);
+        if !t_fmt_ampm_ptr.is_null() {
+            if let Ok(ampm_format) = CStr::from_ptr(t_fmt_ampm_ptr).to_str() {
+                // If T_FMT_AMPM is non-empty and different from T_FMT, locale supports 12-hour
+                if !ampm_format.is_empty() {
+                    if let Some(time_format) = time_fmt_opt {
+                        if ampm_format != time_format {
+                            return true;
+                        }
+                    } else {
+                        return true;
+                    }
+                }
+            }
+        }
+    }
+
+        // Default to 24-hour format if we can't determine
+        false
+    }
+}
+
+cfg_langinfo! {
+    /// Detects whether the current locale prefers 12-hour or 24-hour time format
+    /// Results are cached for performance
+    pub fn uses_12_hour_format() -> bool {
+        *TIME_FORMAT_CACHE.get_or_init(detect_12_hour_format)
+    }
+
+    /// Cached default format string
+    static DEFAULT_FORMAT_CACHE: OnceLock<&'static str> = OnceLock::new();
+
+    /// Get the locale-appropriate default format string for date output
+    /// This respects the locale's preference for 12-hour vs 24-hour time
+    /// Results are cached for performance (following uucore patterns)
+    pub fn get_locale_default_format() -> &'static str {
+        DEFAULT_FORMAT_CACHE.get_or_init(|| {
+            if uses_12_hour_format() {
+                // Use 12-hour format with AM/PM
+                "%a %b %e %r %Z %Y"
+            } else {
+                // Use 24-hour format
+                "%a %b %e %X %Z %Y"
+            }
+        })
+    }
+}
+
+/// On platforms without nl_langinfo support, use 24-hour format by default
+#[cfg(not(any(
+    target_os = "linux",
+    target_vendor = "apple",
+    target_os = "freebsd",
+    target_os = "netbsd",
+    target_os = "openbsd",
+    target_os = "dragonfly"
+)))]
+pub fn get_locale_default_format() -> &'static str {
+    "%a %b %e %X %Z %Y"
+}
+
+#[cfg(test)]
+mod tests {
+    cfg_langinfo! {
+        use super::*;
+
+        #[test]
+        fn test_locale_detection() {
+            // Just verify the function doesn't panic
+            let _ = uses_12_hour_format();
+            let _ = get_locale_default_format();
+        }
+
+        #[test]
+        fn test_default_format_contains_valid_codes() {
+            let format = get_locale_default_format();
+            assert!(format.contains("%a")); // abbreviated weekday
+            assert!(format.contains("%b")); // abbreviated month
+            assert!(format.contains("%Y")); // year
+            assert!(format.contains("%Z")); // timezone
+        }
+    }
+}

tests/by-util/test_date.rs

@@ -1071,3 +1071,58 @@ fn test_date_military_timezone_with_offset_variations() {
             .stdout_is(format!("{expected}\n"));
     }
 }
+
+// Locale-aware hour formatting tests
+#[test]
+#[cfg(unix)]
+fn test_date_locale_hour_c_locale() {
+    // C locale should use 24-hour format
+    new_ucmd!()
+        .env("LC_ALL", "C")
+        .env("TZ", "UTC")
+        .arg("-d")
+        .arg("2025-10-11T13:00")
+        .succeeds()
+        .stdout_contains("13:00");
+}
+
+#[test]
+#[cfg(any(
+    target_os = "linux",
+    target_vendor = "apple",
+    target_os = "freebsd",
+    target_os = "netbsd",
+    target_os = "openbsd",
+    target_os = "dragonfly"
+))]
+fn test_date_locale_hour_en_us() {
+    // en_US locale typically uses 12-hour format when available
+    // Note: If locale is not installed on system, falls back to C locale (24-hour)
+    let result = new_ucmd!()
+        .env("LC_ALL", "en_US.UTF-8")
+        .env("TZ", "UTC")
+        .arg("-d")
+        .arg("2025-10-11T13:00")
+        .succeeds();
+
+    let stdout = result.stdout_str();
+    // Accept either 12-hour (if locale available) or 24-hour (if locale unavailable)
+    // The important part is that the code doesn't crash and handles locale detection gracefully
+    assert!(
+        stdout.contains("1:00") || stdout.contains("13:00"),
+        "date output should contain either 1:00 (12-hour) or 13:00 (24-hour), got: {stdout}"
+    );
+}
+
+#[test]
+fn test_date_explicit_format_overrides_locale() {
+    // Explicit format should override locale preferences
+    new_ucmd!()
+        .env("LC_ALL", "en_US.UTF-8")
+        .env("TZ", "UTC")
+        .arg("-d")
+        .arg("2025-10-11T13:00")
+        .arg("+%H:%M")
+        .succeeds()
+        .stdout_is("13:00\n");
+}