diff --git a/CHANGELOG.next.toml b/CHANGELOG.next.toml
index 73c59508ae..24aba1a457 100644
--- a/CHANGELOG.next.toml
+++ b/CHANGELOG.next.toml
@@ -150,3 +150,9 @@ message = "Fix regression with redacting sensitive HTTP response bodies."
 references = ["smithy-rs#2926", "smithy-rs#2972"]
 meta = { "breaking" = false, "tada" = false, "bug" = true, "target" = "client" }
 author = "ysaito1001"
+
+[[smithy-rs]]
+message = "Omit fractional seconds from `date-time` format."
+references = ["smithy-rs#2831"]
+meta = { "breaking" = false, "tada" = false, "bug" = true, "target" = "all" }
+author = "rschmitt"
diff --git a/rust-runtime/aws-smithy-json/src/serialize.rs b/rust-runtime/aws-smithy-json/src/serialize.rs
index 744028efe4..dce807cc0a 100644
--- a/rust-runtime/aws-smithy-json/src/serialize.rs
+++ b/rust-runtime/aws-smithy-json/src/serialize.rs
@@ -309,7 +309,7 @@ mod tests {
         object.finish();
 
         assert_eq!(
-            r#"{"epoch_seconds":5.2,"date_time":"2021-05-24T15:34:50.123Z","http_date":"Wed, 21 Oct 2015 07:28:00 GMT"}"#,
+            r#"{"epoch_seconds":5.2,"date_time":"2021-05-24T15:34:50Z","http_date":"Wed, 21 Oct 2015 07:28:00 GMT"}"#,
             &output,
         )
     }
@@ -340,7 +340,7 @@ mod tests {
         array.finish();
 
         assert_eq!(
-            r#"[5.2,"2021-05-24T15:34:50.123Z","Wed, 21 Oct 2015 07:28:00 GMT"]"#,
+            r#"[5.2,"2021-05-24T15:34:50Z","Wed, 21 Oct 2015 07:28:00 GMT"]"#,
             &output,
         )
     }
diff --git a/rust-runtime/aws-smithy-query/src/lib.rs b/rust-runtime/aws-smithy-query/src/lib.rs
index 9ce7799718..fc09c6151e 100644
--- a/rust-runtime/aws-smithy-query/src/lib.rs
+++ b/rust-runtime/aws-smithy-query/src/lib.rs
@@ -383,7 +383,7 @@ mod tests {
             "Action=SomeAction\
             &Version=1.0\
             &epoch_seconds=5.2\
-            &date_time=2021-05-24T15%3A34%3A50.123Z\
+            &date_time=2021-05-24T15%3A34%3A50Z\
             &http_date=Wed%2C%2021%20Oct%202015%2007%3A28%3A00%20GMT\
             ",
             out
diff --git a/rust-runtime/aws-smithy-types/src/date_time/format.rs b/rust-runtime/aws-smithy-types/src/date_time/format.rs
index 7253dec1ac..516f04a6b4 100644
--- a/rust-runtime/aws-smithy-types/src/date_time/format.rs
+++ b/rust-runtime/aws-smithy-types/src/date_time/format.rs
@@ -452,7 +452,7 @@ pub(crate) mod rfc3339 {
             )
             .into()
         }
-        let (year, month, day, hour, minute, second, micros) = {
+        let (year, month, day, hour, minute, second) = {
             let s = OffsetDateTime::from_unix_timestamp_nanos(date_time.as_nanos())
                 .map_err(out_of_range)?;
             (
@@ -462,7 +462,6 @@ pub(crate) mod rfc3339 {
                 s.hour(),
                 s.minute(),
                 s.second(),
-                s.microsecond(),
             )
         };
 
@@ -479,26 +478,9 @@ pub(crate) mod rfc3339 {
             year, month, day, hour, minute, second
         )
         .unwrap();
-        format_subsecond_fraction(&mut out, micros);
         out.push('Z');
         Ok(out)
     }
-
-    /// Formats sub-second fraction for RFC-3339 (including the '.').
-    /// Expects to be called with a number of `micros` between 0 and 999_999 inclusive.
-    fn format_subsecond_fraction(into: &mut String, micros: u32) {
-        debug_assert!(micros < 1_000_000);
-        if micros > 0 {
-            into.push('.');
-            let (mut remaining, mut place) = (micros, 100_000);
-            while remaining > 0 {
-                let digit = (remaining / place) % 10;
-                into.push(char::from(b'0' + (digit as u8)));
-                remaining -= digit * place;
-                place /= 10;
-            }
-        }
-    }
 }
 
 #[cfg(test)]
@@ -623,7 +605,7 @@ mod tests {
             rfc3339::format(&DateTime::from_secs(-62_135_596_800)).unwrap()
         );
         assert_eq!(
-            "9999-12-31T23:59:59.999999Z",
+            "9999-12-31T23:59:59Z",
             rfc3339::format(&DateTime::from_secs_and_nanos(253402300799, 999_999_999)).unwrap()
         );
 
diff --git a/rust-runtime/aws-smithy-types/src/date_time/mod.rs b/rust-runtime/aws-smithy-types/src/date_time/mod.rs
index b5e740cf5b..4701e610df 100644
--- a/rust-runtime/aws-smithy-types/src/date_time/mod.rs
+++ b/rust-runtime/aws-smithy-types/src/date_time/mod.rs
@@ -388,7 +388,7 @@ mod test {
         let date_time = DateTime::from_fractional_secs(1576540098, 0.52);
         assert_eq!(
             date_time.fmt(Format::DateTime).unwrap(),
-            "2019-12-16T23:48:18.52Z"
+            "2019-12-16T23:48:18Z"
         );
         assert_eq!(
             date_time.fmt(Format::EpochSeconds).unwrap(),
diff --git a/rust-runtime/aws-smithy-types/test_data/date_time_format_test_suite.json b/rust-runtime/aws-smithy-types/test_data/date_time_format_test_suite.json
index a79e23e4aa..88c4f33779 100644
--- a/rust-runtime/aws-smithy-types/test_data/date_time_format_test_suite.json
+++ b/rust-runtime/aws-smithy-types/test_data/date_time_format_test_suite.json
@@ -1943,7 +1943,7 @@
           "canonical_seconds": "-62133482201",
           "canonical_nanos": 123456789,
           "error": false,
-          "smithy_format_value": "0001-01-25T11:23:19.123456Z"
+          "smithy_format_value": "0001-01-25T11:23:19Z"
       },
       {
           "iso8601": "0001-02-01T12:24:12Z",
@@ -1978,49 +1978,49 @@
           "canonical_seconds": "-62120071876",
           "canonical_nanos": 1000,
           "error": false,
-          "smithy_format_value": "0001-06-29T16:28:44.000001Z"
+          "smithy_format_value": "0001-06-29T16:28:44Z"
       },
       {
           "iso8601": "0001-07-31T17:29:37.00001Z",
           "canonical_seconds": "-62117303423",
           "canonical_nanos": 10000,
           "error": false,
-          "smithy_format_value": "0001-07-31T17:29:37.00001Z"
+          "smithy_format_value": "0001-07-31T17:29:37Z"
       },
       {
           "iso8601": "0001-08-01T18:30:30.0001Z",
           "canonical_seconds": "-62117213370",
           "canonical_nanos": 100000,
           "error": false,
-          "smithy_format_value": "0001-08-01T18:30:30.0001Z"
+          "smithy_format_value": "0001-08-01T18:30:30Z"
       },
       {
           "iso8601": "0001-09-02T19:31:23.001Z",
           "canonical_seconds": "-62114444917",
           "canonical_nanos": 1000000,
           "error": false,
-          "smithy_format_value": "0001-09-02T19:31:23.001Z"
+          "smithy_format_value": "0001-09-02T19:31:23Z"
       },
       {
           "iso8601": "0001-10-03T20:32:16.01Z",
           "canonical_seconds": "-62111762864",
           "canonical_nanos": 10000000,
           "error": false,
-          "smithy_format_value": "0001-10-03T20:32:16.01Z"
+          "smithy_format_value": "0001-10-03T20:32:16Z"
       },
       {
           "iso8601": "0001-11-04T21:33:09.1Z",
           "canonical_seconds": "-62108994411",
           "canonical_nanos": 100000000,
           "error": false,
-          "smithy_format_value": "0001-11-04T21:33:09.1Z"
+          "smithy_format_value": "0001-11-04T21:33:09Z"
       },
       {
           "iso8601": "0001-12-05T22:34:02.123456789Z",
           "canonical_seconds": "-62106312358",
           "canonical_nanos": 123456789,
           "error": false,
-          "smithy_format_value": "0001-12-05T22:34:02.123456Z"
+          "smithy_format_value": "0001-12-05T22:34:02Z"
       },
       {
           "iso8601": "0100-01-06T23:35:55Z",
@@ -2055,49 +2055,49 @@
           "canonical_seconds": "-59000300433",
           "canonical_nanos": 1000,
           "error": false,
-          "smithy_format_value": "0100-05-10T03:39:27.000001Z"
+          "smithy_format_value": "0100-05-10T03:39:27Z"
       },
       {
           "iso8601": "0100-06-11T04:40:20.00001Z",
           "canonical_seconds": "-58997531980",
           "canonical_nanos": 10000,
           "error": false,
-          "smithy_format_value": "0100-06-11T04:40:20.00001Z"
+          "smithy_format_value": "0100-06-11T04:40:20Z"
       },
       {
           "iso8601": "0100-07-12T05:41:13.0001Z",
           "canonical_seconds": "-58994849927",
           "canonical_nanos": 100000,
           "error": false,
-          "smithy_format_value": "0100-07-12T05:41:13.0001Z"
+          "smithy_format_value": "0100-07-12T05:41:13Z"
       },
       {
           "iso8601": "0100-08-13T06:42:06.001Z",
           "canonical_seconds": "-58992081474",
           "canonical_nanos": 1000000,
           "error": false,
-          "smithy_format_value": "0100-08-13T06:42:06.001Z"
+          "smithy_format_value": "0100-08-13T06:42:06Z"
       },
       {
           "iso8601": "0100-09-14T07:43:59.01Z",
           "canonical_seconds": "-58989312961",
           "canonical_nanos": 10000000,
           "error": false,
-          "smithy_format_value": "0100-09-14T07:43:59.01Z"
+          "smithy_format_value": "0100-09-14T07:43:59Z"
       },
       {
           "iso8601": "0100-10-15T08:44:52.1Z",
           "canonical_seconds": "-58986630908",
           "canonical_nanos": 100000000,
           "error": false,
-          "smithy_format_value": "0100-10-15T08:44:52.1Z"
+          "smithy_format_value": "0100-10-15T08:44:52Z"
       },
       {
           "iso8601": "0100-11-16T09:45:45.123456789Z",
           "canonical_seconds": "-58983862455",
           "canonical_nanos": 123456789,
           "error": false,
-          "smithy_format_value": "0100-11-16T09:45:45.123456Z"
+          "smithy_format_value": "0100-11-16T09:45:45Z"
       },
       {
           "iso8601": "0100-12-17T10:46:38Z",
@@ -2111,7 +2111,7 @@
           "canonical_seconds": "-12175833601",
           "canonical_nanos": 999000000,
           "error": false,
-          "smithy_format_value": "1584-02-29T23:59:59.999Z"
+          "smithy_format_value": "1584-02-29T23:59:59Z"
       },
       {
           "iso8601": "1969-01-18T11:47:31.000000001Z",
@@ -2139,49 +2139,49 @@
           "canonical_seconds": "-21978590",
           "canonical_nanos": 1000,
           "error": false,
-          "smithy_format_value": "1969-04-21T14:50:10.000001Z"
+          "smithy_format_value": "1969-04-21T14:50:10Z"
       },
       {
           "iso8601": "1969-05-22T15:51:03.00001Z",
           "canonical_seconds": "-19296537",
           "canonical_nanos": 10000,
           "error": false,
-          "smithy_format_value": "1969-05-22T15:51:03.00001Z"
+          "smithy_format_value": "1969-05-22T15:51:03Z"
       },
       {
           "iso8601": "1969-06-23T16:52:56.0001Z",
           "canonical_seconds": "-16528024",
           "canonical_nanos": 100000,
           "error": false,
-          "smithy_format_value": "1969-06-23T16:52:56.0001Z"
+          "smithy_format_value": "1969-06-23T16:52:56Z"
       },
       {
           "iso8601": "1969-07-24T17:53:49.001Z",
           "canonical_seconds": "-13845971",
           "canonical_nanos": 1000000,
           "error": false,
-          "smithy_format_value": "1969-07-24T17:53:49.001Z"
+          "smithy_format_value": "1969-07-24T17:53:49Z"
       },
       {
           "iso8601": "1969-08-25T18:54:42.01Z",
           "canonical_seconds": "-11077518",
           "canonical_nanos": 10000000,
           "error": false,
-          "smithy_format_value": "1969-08-25T18:54:42.01Z"
+          "smithy_format_value": "1969-08-25T18:54:42Z"
       },
       {
           "iso8601": "1969-09-26T19:55:35.1Z",
           "canonical_seconds": "-8309065",
           "canonical_nanos": 100000000,
           "error": false,
-          "smithy_format_value": "1969-09-26T19:55:35.1Z"
+          "smithy_format_value": "1969-09-26T19:55:35Z"
       },
       {
           "iso8601": "1969-10-27T20:56:28.123456789Z",
           "canonical_seconds": "-5627012",
           "canonical_nanos": 123456789,
           "error": false,
-          "smithy_format_value": "1969-10-27T20:56:28.123456Z"
+          "smithy_format_value": "1969-10-27T20:56:28Z"
       },
       {
           "iso8601": "1969-11-28T21:57:21Z",
@@ -2216,49 +2216,49 @@
           "canonical_seconds": "5101313",
           "canonical_nanos": 1000,
           "error": false,
-          "smithy_format_value": "1970-03-01T01:01:53.000001Z"
+          "smithy_format_value": "1970-03-01T01:01:53Z"
       },
       {
           "iso8601": "1970-04-03T02:02:46.00001Z",
           "canonical_seconds": "7956166",
           "canonical_nanos": 10000,
           "error": false,
-          "smithy_format_value": "1970-04-03T02:02:46.00001Z"
+          "smithy_format_value": "1970-04-03T02:02:46Z"
       },
       {
           "iso8601": "1970-05-03T03:03:39.0001Z",
           "canonical_seconds": "10551819",
           "canonical_nanos": 100000,
           "error": false,
-          "smithy_format_value": "1970-05-03T03:03:39.0001Z"
+          "smithy_format_value": "1970-05-03T03:03:39Z"
       },
       {
           "iso8601": "1970-06-05T04:04:32.001Z",
           "canonical_seconds": "13406672",
           "canonical_nanos": 1000000,
           "error": false,
-          "smithy_format_value": "1970-06-05T04:04:32.001Z"
+          "smithy_format_value": "1970-06-05T04:04:32Z"
       },
       {
           "iso8601": "1970-07-05T05:05:25.01Z",
           "canonical_seconds": "16002325",
           "canonical_nanos": 10000000,
           "error": false,
-          "smithy_format_value": "1970-07-05T05:05:25.01Z"
+          "smithy_format_value": "1970-07-05T05:05:25Z"
       },
       {
           "iso8601": "1970-08-06T06:06:18.1Z",
           "canonical_seconds": "18770778",
           "canonical_nanos": 100000000,
           "error": false,
-          "smithy_format_value": "1970-08-06T06:06:18.1Z"
+          "smithy_format_value": "1970-08-06T06:06:18Z"
       },
       {
           "iso8601": "1970-09-08T07:07:11.123456789Z",
           "canonical_seconds": "21625631",
           "canonical_nanos": 123456789,
           "error": false,
-          "smithy_format_value": "1970-09-08T07:07:11.123456Z"
+          "smithy_format_value": "1970-09-08T07:07:11Z"
       },
       {
           "iso8601": "1970-10-08T08:08:04Z",
@@ -2286,7 +2286,7 @@
           "canonical_seconds": "1078099199",
           "canonical_nanos": 999000000,
           "error": false,
-          "smithy_format_value": "2004-02-29T23:59:59.999Z"
+          "smithy_format_value": "2004-02-29T23:59:59Z"
       },
       {
           "iso8601": "2037-01-11T11:11:43.0000001Z",
@@ -2300,49 +2300,49 @@
           "canonical_seconds": "2118831156",
           "canonical_nanos": 1000,
           "error": false,
-          "smithy_format_value": "2037-02-21T12:12:36.000001Z"
+          "smithy_format_value": "2037-02-21T12:12:36Z"
       },
       {
           "iso8601": "2037-03-13T13:13:29.00001Z",
           "canonical_seconds": "2120562809",
           "canonical_nanos": 10000,
           "error": false,
-          "smithy_format_value": "2037-03-13T13:13:29.00001Z"
+          "smithy_format_value": "2037-03-13T13:13:29Z"
       },
       {
           "iso8601": "2037-04-15T14:14:22.0001Z",
           "canonical_seconds": "2123417662",
           "canonical_nanos": 100000,
           "error": false,
-          "smithy_format_value": "2037-04-15T14:14:22.0001Z"
+          "smithy_format_value": "2037-04-15T14:14:22Z"
       },
       {
           "iso8601": "2037-05-15T15:15:15.001Z",
           "canonical_seconds": "2126013315",
           "canonical_nanos": 1000000,
           "error": false,
-          "smithy_format_value": "2037-05-15T15:15:15.001Z"
+          "smithy_format_value": "2037-05-15T15:15:15Z"
       },
       {
           "iso8601": "2037-06-17T16:16:08.01Z",
           "canonical_seconds": "2128868168",
           "canonical_nanos": 10000000,
           "error": false,
-          "smithy_format_value": "2037-06-17T16:16:08.01Z"
+          "smithy_format_value": "2037-06-17T16:16:08Z"
       },
       {
           "iso8601": "2037-07-17T17:17:01.1Z",
           "canonical_seconds": "2131463821",
           "canonical_nanos": 100000000,
           "error": false,
-          "smithy_format_value": "2037-07-17T17:17:01.1Z"
+          "smithy_format_value": "2037-07-17T17:17:01Z"
       },
       {
           "iso8601": "2037-08-18T18:18:54.123456789Z",
           "canonical_seconds": "2134232334",
           "canonical_nanos": 123456789,
           "error": false,
-          "smithy_format_value": "2037-08-18T18:18:54.123456Z"
+          "smithy_format_value": "2037-08-18T18:18:54Z"
       },
       {
           "iso8601": "2037-09-20T19:19:47Z",
@@ -2377,49 +2377,49 @@
           "canonical_seconds": "2147901799",
           "canonical_nanos": 1000,
           "error": false,
-          "smithy_format_value": "2038-01-23T23:23:19.000001Z"
+          "smithy_format_value": "2038-01-23T23:23:19Z"
       },
       {
           "iso8601": "2038-02-05T00:24:12.00001Z",
           "canonical_seconds": "2148942252",
           "canonical_nanos": 10000,
           "error": false,
-          "smithy_format_value": "2038-02-05T00:24:12.00001Z"
+          "smithy_format_value": "2038-02-05T00:24:12Z"
       },
       {
           "iso8601": "2038-03-25T01:25:05.0001Z",
           "canonical_seconds": "2153093105",
           "canonical_nanos": 100000,
           "error": false,
-          "smithy_format_value": "2038-03-25T01:25:05.0001Z"
+          "smithy_format_value": "2038-03-25T01:25:05Z"
       },
       {
           "iso8601": "2038-04-27T02:26:58.001Z",
           "canonical_seconds": "2155948018",
           "canonical_nanos": 1000000,
           "error": false,
-          "smithy_format_value": "2038-04-27T02:26:58.001Z"
+          "smithy_format_value": "2038-04-27T02:26:58Z"
       },
       {
           "iso8601": "2038-05-27T03:27:51.01Z",
           "canonical_seconds": "2158543671",
           "canonical_nanos": 10000000,
           "error": false,
-          "smithy_format_value": "2038-05-27T03:27:51.01Z"
+          "smithy_format_value": "2038-05-27T03:27:51Z"
       },
       {
           "iso8601": "2038-06-29T04:28:44.1Z",
           "canonical_seconds": "2161398524",
           "canonical_nanos": 100000000,
           "error": false,
-          "smithy_format_value": "2038-06-29T04:28:44.1Z"
+          "smithy_format_value": "2038-06-29T04:28:44Z"
       },
       {
           "iso8601": "2038-07-29T05:29:37.123456789Z",
           "canonical_seconds": "2163994177",
           "canonical_nanos": 123456789,
           "error": false,
-          "smithy_format_value": "2038-07-29T05:29:37.123456Z"
+          "smithy_format_value": "2038-07-29T05:29:37Z"
       },
       {
           "iso8601": "2038-08-30T06:30:30Z",
@@ -2454,49 +2454,49 @@
           "canonical_seconds": "2174985242",
           "canonical_nanos": 1000,
           "error": false,
-          "smithy_format_value": "2038-12-03T10:34:02.000001Z"
+          "smithy_format_value": "2038-12-03T10:34:02Z"
       },
       {
           "iso8601": "9999-01-04T11:35:55.00001Z",
           "canonical_seconds": "253371065755",
           "canonical_nanos": 10000,
           "error": false,
-          "smithy_format_value": "9999-01-04T11:35:55.00001Z"
+          "smithy_format_value": "9999-01-04T11:35:55Z"
       },
       {
           "iso8601": "9999-02-17T12:36:48.0001Z",
           "canonical_seconds": "253374871008",
           "canonical_nanos": 100000,
           "error": false,
-          "smithy_format_value": "9999-02-17T12:36:48.0001Z"
+          "smithy_format_value": "9999-02-17T12:36:48Z"
       },
       {
           "iso8601": "9999-03-06T13:37:41.001Z",
           "canonical_seconds": "253376343461",
           "canonical_nanos": 1000000,
           "error": false,
-          "smithy_format_value": "9999-03-06T13:37:41.001Z"
+          "smithy_format_value": "9999-03-06T13:37:41Z"
       },
       {
           "iso8601": "9999-04-09T14:38:34.01Z",
           "canonical_seconds": "253379284714",
           "canonical_nanos": 10000000,
           "error": false,
-          "smithy_format_value": "9999-04-09T14:38:34.01Z"
+          "smithy_format_value": "9999-04-09T14:38:34Z"
       },
       {
           "iso8601": "9999-05-08T15:39:27.1Z",
           "canonical_seconds": "253381793967",
           "canonical_nanos": 100000000,
           "error": false,
-          "smithy_format_value": "9999-05-08T15:39:27.1Z"
+          "smithy_format_value": "9999-05-08T15:39:27Z"
       },
       {
           "iso8601": "9999-06-11T16:40:20.123456789Z",
           "canonical_seconds": "253384735220",
           "canonical_nanos": 123456789,
           "error": false,
-          "smithy_format_value": "9999-06-11T16:40:20.123456Z"
+          "smithy_format_value": "9999-06-11T16:40:20Z"
       },
       {
           "iso8601": "9999-07-10T17:41:13Z",
@@ -2531,14 +2531,14 @@
           "canonical_seconds": "253398404745",
           "canonical_nanos": 1000,
           "error": false,
-          "smithy_format_value": "9999-11-16T21:45:45.000001Z"
+          "smithy_format_value": "9999-11-16T21:45:45Z"
       },
       {
           "iso8601": "9999-12-15T22:46:38.00001Z",
           "canonical_seconds": "253400913998",
           "canonical_nanos": 10000,
           "error": false,
-          "smithy_format_value": "9999-12-15T22:46:38.00001Z"
+          "smithy_format_value": "9999-12-15T22:46:38Z"
       }
   ],
   "parse_epoch_seconds": [