DateTime.now()。toUtc()。difference和DateTime.now()。在飞镖中，差异产生相同的结果

Question

基本上是标题。我想得到日期时间和当前时间之间的差异。日期以UTC为单位，我首先将当前时间转换为UTC，然后找到差异。但在两种情况下，转换为UTC或不转换为UTC的差异是相同的，但不应如此。

 print(DateTime.now().toUtc());
    print(DateTime.now());
    print(DateTime.now().toUtc()
        .difference(dateFormat.parse(comment.postedOn))
        .inMinutes);
    print( DateTime.now()
        .difference(dateFormat.parse(comment.postedOn))
        .inMinutes);

结果是

内/扑动 （20796）： 2020-07-01 15：33：46.221975Z

I/颤动 （20796）： 2020-07-01 21：03：46.222207

I/颤振 （20796）： 361

I/颤振 （20796）： 361

Answer 1

在对Dart SDK的源代码进行一些挖掘之后，它看起来像是＜code＞毫秒历元DateTime值和本地DateTime值的差异相同，因为它们都表示相同的时间。

final now = DateTime.now();
final utc = now.toUtc();
final date = DateTime.parse('2020-07-01T19:00:00Z');

print(utc);
print(now);
print(date);
print(utc.microsecondsSinceEpoch);
print(now.microsecondsSinceEpoch);
print(date.microsecondsSinceEpoch);

print(utc.difference(date).inMinutes);
print(now.difference(date).inMinutes);

2020-07-01 16:26:54.464Z

2020-07-01 17:26:54.464

2020-07-01 19:00:00.000Z

1593620814464000

1593620814464000

1593630000000000

-153

-153

./sdk/lib/core/date_time.dart

/**
 * The value of this DateTime.
 *
 * The content of this field is implementation dependent. On JavaScript it is
 * equal to [millisecondsSinceEpoch]. On the VM it is equal to
 * [microsecondsSinceEpoch].
 */
final int _value;

js_runtime/自由/core_patch飞镖

@patch
Duration difference(DateTime other) {
  return new Duration(milliseconds: _value - other._value);
}

./vm/lib/date_patch.dart

@patch
Duration difference(DateTime other) {
  return new Duration(microseconds: _value - other._value);
}

如果您不想要这种默认行为，那么您可以检查< code>DateTime值< code>isUtc是否存在。如果值不是UTC，那么您可以添加< code>timeZoneOffset来实现您想要的行为。

extension DateTimeExtensions on DateTime {
  Duration differenceTimeZoneOffset(DateTime other) {
    if (this.isUtc) {
      return this.difference(other);
    } else {
      return this.add(this.timeZoneOffset).difference(other);
    }
  }
}

final now = DateTime.now();
final utc = now.toUtc();
final date = utc.add(Duration(hours: 1));

print(utc);
print(now);
print(date);
print(utc.microsecondsSinceEpoch);
print(now.microsecondsSinceEpoch);
print(date.microsecondsSinceEpoch);

print(utc.differenceTimeZoneOffset(date).inMinutes);
print(now.differenceTimeZoneOffset(date).inMinutes);

2020-07-01 17:24:54.923Z

2020-07-01 18:24:54.923

2020-07-01 18:24:54.923Z

1593624294923000

1593624294923000

1593627894923000

-60

0