dmesg: support iso time-format.

src/uu/dmesg/src/dmesg.rs

@@ -158,6 +158,9 @@ impl Dmesg<'_> {
                     TimeFormat::Ctime => {
                         print!("[{}] ", time_formatter::ctime(record.timestamp_us))
                     }
+                    TimeFormat::Iso => {
+                        print!("{} ", time_formatter::iso(record.timestamp_us))
+                    }
                     TimeFormat::Raw => {
                         print!("[{}] ", time_formatter::raw(record.timestamp_us))
                     }

src/uu/dmesg/src/time_formatter.rs

@@ -16,6 +16,13 @@ pub fn ctime(timestamp_us: u64) -> String {
     date_time.format("%a %b %d %H:%M:%S %Y").to_string()
 }
 
+pub fn iso(timestamp_us: u64) -> String {
+    let date_time = boot_time()
+        .checked_add_signed(TimeDelta::microseconds(timestamp_us as i64))
+        .unwrap();
+    date_time.format("%Y-%m-%dT%H:%M:%S,%6f%:z").to_string()
+}
+
 static BOOT_TIME: OnceLock<DateTime<FixedOffset>> = OnceLock::new();
 
 #[cfg(feature = "fixed-boot-time")]