在Apache Beam中使用PubSub文件处理时，可以使用PubsubIO.Read.timestampLabel()方法来指定消息中的时间戳字段。然后，可以使用ParDo转换来计算时间延迟。

下面是一个示例代码，演示了如何在Apache Beam中处理PubSub文件并计算时间延迟：

import org.apache.beam.sdk.Pipeline;
import org.apache.beam.sdk.io.gcp.pubsub.PubsubIO;
import org.apache.beam.sdk.io.gcp.pubsub.PubsubMessage;
import org.apache.beam.sdk.options.PipelineOptions;
import org.apache.beam.sdk.options.PipelineOptionsFactory;
import org.apache.beam.sdk.transforms.DoFn;
import org.apache.beam.sdk.transforms.ParDo;
import org.apache.beam.sdk.transforms.windowing.BoundedWindow;
import org.apache.beam.sdk.transforms.windowing.IntervalWindow;
import org.apache.beam.sdk.transforms.windowing.Window;
import org.apache.beam.sdk.values.PCollection;
import org.joda.time.Instant;

public class PubSubFileProcessing {
  public static void main(String[] args) {
    // 创建PipelineOptions
    PipelineOptions options = PipelineOptionsFactory.create();

    // 创建Pipeline
    Pipeline pipeline = Pipeline.create(options);

    // 从PubSub读取数据
    PCollection messages = pipeline
        .apply("Read from PubSub", PubsubIO.readMessages().fromSubscription("projects/{project}/subscriptions/{subscription}"));

    // 提取时间戳字段并计算时间延迟
    PCollection timeDelays = messages
        .apply("Extract timestamp", ParDo.of(new ExtractTimestampFn()))
        .apply("Calculate time delay", ParDo.of(new CalculateTimeDelayFn()));

    // 输出时间延迟结果
    timeDelays.apply("Print time delays", ParDo.of(new PrintTimeDelaysFn()));

    // 运行Pipeline
    pipeline.run();
  }

  // 提取时间戳字段的DoFn
  static class ExtractTimestampFn extends DoFn {
    @ProcessElement
    public void processElement(ProcessContext c) {
      PubsubMessage message = c.element();
      // 从消息中提取时间戳字段
      Instant timestamp = new Instant(message.getAttribute("timestamp"));
      c.output(timestamp);
    }
  }

  // 计算时间延迟的DoFn
  static class CalculateTimeDelayFn extends DoFn {
    @ProcessElement
    public void processElement(ProcessContext c, BoundedWindow window) {
      Instant elementTimestamp = c.element();
      // 计算时间延迟
      IntervalWindow windowBounds = (IntervalWindow) window;
      long timeDelay = windowBounds.start().getMillis() - elementTimestamp.getMillis();
      c.output(timeDelay);
    }
  }

  // 输出时间延迟结果的DoFn
  static class PrintTimeDelaysFn extends DoFn {
    @ProcessElement
    public void processElement(ProcessContext c) {
      System.out.println("Time delay: " + c.element());
    }
  }
}

请注意，这只是一个示例代码，你需要根据你的具体需求进行修改和适配。在代码中，你需要将{project}和{subscription}替换为你的GCP项目和PubSub订阅的相关信息。