feat(nextcloud): add date time utils

Signed-off-by: Nikolas Rimikis <leptopoda@users.noreply.github.com>

Author: Leptopoda

packages/nextcloud/lib/src/utils/date_time.dart

@@ -0,0 +1,30 @@
+/// Common methods to work with [DateTime] data.
+extension DateTimeUtils on DateTime {
+  /// 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:[secondsSinceEpoch]Z in the given
+  /// time zone (local or UTC).
+  /// ```dart
+  /// final newYearsDay =
+  ///     DateTimeUtils.fromSecondsSinceEpoch(1641031200, isUtc:true);
+  /// print(newYearsDay); // 2022-01-01 10:00:00.000Z
+  /// ```
+  static DateTime fromSecondsSinceEpoch(int secondsSinceEpoch, {bool isUtc = false}) {
+    final millisecondsSinceEpoch = secondsSinceEpoch * Duration.millisecondsPerSecond;
+
+    return DateTime.fromMillisecondsSinceEpoch(millisecondsSinceEpoch, isUtc: 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
+  /// 8,640,000,000,000s (100,000,000 days) from the Unix epoch.
+  /// In other words: `secondsSinceEpoch.abs() <= 8640000000000`.
+  int get secondsSinceEpoch => millisecondsSinceEpoch ~/ Duration.millisecondsPerSecond;
+}

packages/nextcloud/lib/utils.dart

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+/// This library contains various utility methods for working with nextcloud APIs.
+library;
+
+export 'src/utils/date_time.dart';

packages/nextcloud/test/utils_test.dart

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+import 'package:nextcloud/src/utils/date_time.dart';
+import 'package:test/test.dart';
+
+void main() {
+  test('DateTimeUtils', () {
+    final date = DateTime.utc(2016, 6, 2, 23, 53, 45, 243, 234);
+
+    expect(date.secondsSinceEpoch, 1464911625);
+    expect(date.microsecondsSinceEpoch, 1464911625243234);
+
+    expect(DateTimeUtils.fromSecondsSinceEpoch(1464911625, isUtc: true), date.copyWith(millisecond: 0, microsecond: 0));
+    expect(DateTimeUtils.fromSecondsSinceEpoch(1464911625, isUtc: true).microsecondsSinceEpoch, 1464911625000000);
+  });
+}