New API: Allow creating timestamp with custom time.

dd-trace/src/main/java/datadog/trace/tracer/Clock.java

@@ -47,6 +47,15 @@ class Clock {
     return new Timestamp(this);
   }
 
+  /**
+   * Create new timestamp instance for current time.
+   *
+   * @return new timestamp capturing current time.
+   */
+  public Timestamp createTimestampForTime(final long time, final TimeUnit unit) {
+    return new Timestamp(this, time, unit);
+  }
+
   /**
    * Get the current nanos ticks (i.e. System.nanoTime()), this method can't be use for date
    * accuracy (only duration calculations).

dd-trace/src/main/java/datadog/trace/tracer/Timestamp.java

@@ -3,6 +3,7 @@ package datadog.trace.tracer;
 import static java.lang.Math.max;
 
 import com.fasterxml.jackson.annotation.JsonValue;
+import java.util.concurrent.TimeUnit;
 import lombok.EqualsAndHashCode;
 
 /**
@@ -26,6 +27,20 @@ public class Timestamp {
     nanoTicks = clock.nanoTicks();
   }
 
+  /**
+   * Create timestamp for a given clock and given start time and precision
+   *
+   * @param clock clock instance
+   */
+  Timestamp(final Clock clock, final long time, final TimeUnit unit) {
+    this.clock = clock;
+    final long currentTime = clock.epochTimeNano();
+    final long currentTick = clock.nanoTicks();
+    final long desiredTime = unit.toNanos(time);
+    final long offset = currentTime - desiredTime;
+    nanoTicks = currentTick - offset;
+  }
+
   /** @return clock instance used by this timestamp */
   Clock getClock() {
     return clock;

dd-trace/src/main/java/datadog/trace/tracer/Tracer.java

@@ -9,6 +9,7 @@ import java.io.IOException;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
+import java.util.concurrent.TimeUnit;
 import lombok.Builder;
 import lombok.extern.slf4j.Slf4j;
 
@@ -74,6 +75,14 @@ public class Tracer implements Closeable {
     return new Clock(this).createCurrentTimestamp();
   }
 
+  /**
+   * @return timestamp for given time. Note: this is mainly useful when there is no 'current' trace.
+   *     If there is 'current' trace already then one should use it to get timestamps.
+   */
+  public Timestamp createTimestampForTime(final long time, final TimeUnit unit) {
+    return new Clock(this).createTimestampForTime(time, unit);
+  }
+
   /**
    * Construct a new trace using this tracer's settings and return the root span.
    *

dd-trace/src/test/groovy/datadog/trace/tracer/ClockTest.groovy

@@ -43,6 +43,17 @@ class ClockTest extends Specification {
     timestamp.getClock() == clock
   }
 
+  def "test timestamp creation with custom time"() {
+    setup:
+    def clock = new Clock(tracer)
+
+    when:
+    def timestamp = clock.createTimestampForTime(1, TimeUnit.SECONDS)
+
+    then:
+    timestamp.getClock() == clock
+  }
+
   def "test equals"() {
     when:
     EqualsVerifier.forClass(Clock).suppress(Warning.STRICT_INHERITANCE).verify()

dd-trace/src/test/groovy/datadog/trace/tracer/TimestampTest.groovy

@@ -5,6 +5,9 @@ import nl.jqno.equalsverifier.EqualsVerifier
 import nl.jqno.equalsverifier.Warning
 import spock.lang.Specification
 
+import static java.util.concurrent.TimeUnit.MICROSECONDS
+import static java.util.concurrent.TimeUnit.NANOSECONDS
+
 class TimestampTest extends Specification {
 
   private static final long CLOCK_START_TIME = 100
@@ -36,11 +39,33 @@ class TimestampTest extends Specification {
     clock.getStartNanoTicks() >> CLOCK_START_NANO_TICKS
     def timestamp = new Timestamp(clock)
 
+    when:
+    def duration = timestamp.getDuration(FINISH_TIME)
+
+    then:
+    duration == 400
+    0 * tracer._
+  }
+
+  def "test timestamp with custom time"() {
+    setup:
+    clock.nanoTicks() >> CLOCK_NANO_TICKS
+    clock.getStartTimeNano() >> CLOCK_START_TIME
+    clock.getStartNanoTicks() >> CLOCK_START_NANO_TICKS
+    def timestamp = new Timestamp(clock, CLOCK_START_TIME + offset, unit)
+
     when:
     def time = timestamp.getTime()
 
     then:
-    time == 300
+    time == expected
+
+    where:
+    offset | unit         | expected
+    10     | NANOSECONDS  | 410
+    -20    | NANOSECONDS  | 380
+    3      | MICROSECONDS | 103300
+    -4     | MICROSECONDS | 96300
   }
 
   def "test getDuration with literal finish time"() {