[WIP] feat: Add DateTimeSecondsSinceEpoch

sdk/lib/core/date_time.dart

@@ -399,6 +399,25 @@ class DateTime implements Comparable<DateTime> {
   static const int _maxMicrosecondsSinceEpoch =
       _maxMillisecondsSinceEpoch * Duration.microsecondsPerMillisecond;
 
+  /// Constructs a new [DateTime] instance
+  /// with the given [secondsSinceEpoch].
+  ///
+  /// /// If [isUtc] is false then the date is in the local time zone.
+  ///
+  /// The constructed [DateTime] represents
+  /// 1970-01-01T00:00:00Z + [secondsSinceEpoch] seconds in the given
+  /// time zone (local or UTC).
+  /// ```dart
+  /// final newYearsDay =
+  ///     DateTime.fromSecondsSinceEpoch(1641031200, isUtc:true);
+  /// print(newYearsDay); // 2022-01-01 10:00:00.000Z
+  /// ```
+  @Since("3.8")
+  external DateTime.fromSecondsSinceEpoch(
+    int secondsSinceEpoch, {
+    bool isUtc = false,
+  });
+
   /// Constructs a new [DateTime] instance
   /// with the given [millisecondsSinceEpoch].
   ///
@@ -788,6 +807,17 @@ class DateTime implements Comparable<DateTime> {
     bool isUtc,
   );
 
+  /// The number of seconds since
+  /// the "Unix epoch" 1970-01-01T00:00:00Z (UTC).
+  ///
+  /// This value is independent of the time zone.
+  ///
+  /// This value is at most
+  /// 86,400,000,000s. (100,000,000 days) from the Unix epoch.
+  /// In other words: `secondsSinceEpoch.abs() <= 86400000000`.
+  @Since("3.8")
+  external int get secondsSinceEpoch;
+
   /// The number of milliseconds since
   /// the "Unix epoch" 1970-01-01T00:00:00Z (UTC).
   ///

tests/corelib/date_time_seconds_since_epoch_test.dart

@@ -0,0 +1,20 @@
+// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import "package:expect/expect.dart";
+
+/// Tests `DateTime.secondsSinceEpoch`.
+
+testSecondsSinceEpoch(DateTime original, int expectedSeconds) {
+  final result = original.secondsSinceEpoch;
+  Expect.equals(expectedSeconds, result);
+}
+
+void main() {
+  final epoch = DateTime.utc(1970, 1, 1);
+  final oneHourLater = DateTime.utc(1970, 1, 1, 1); // 3600 seconds since epoch
+
+  testSecondsSinceEpoch(epoch, 0);
+  testSecondsSinceEpoch(oneHourLater, 3600);
+}