refactor(c-binding): trim explicit-timestamp change

Drop transitive includes, inline locals in the helper and in the test,
and compress comments to match the surrounding file style.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: adeshkumar1

binding/c/include/datadog/c/tracer.h

@@ -52,9 +52,7 @@ typedef struct {
   const char* service_type;
   const char* environment;
   const char* version;
-  // Wall-clock span start time in nanoseconds since the UNIX epoch.
-  // 0 means "use the current time".
-  int64_t start_time_ns;
+  int64_t start_time_ns;  // UNIX-epoch nanoseconds; 0 = use current time
 } dd_span_options_t;
 
 // Error codes returned by the C binding.
@@ -183,12 +181,10 @@ DD_TRACE_C_API dd_span_t* dd_span_create_child(dd_span_t* span_handle,
 // @param span_handle Span handle
 DD_TRACE_C_API void dd_span_finish(dd_span_t* span_handle);
 
-// Finish a span using an explicit end time given as wall-clock nanoseconds
-// since the UNIX epoch. If end_time_ns is 0, behaves like dd_span_finish
-// (uses the current time). No-op if span_handle is NULL.
+// Finish a span using an explicit end time. No-op if span_handle is NULL.
 //
 // @param span_handle Span handle
-// @param end_time_ns Wall-clock end time in nanoseconds since the UNIX epoch
+// @param end_time_ns Wall-clock end time (UNIX-epoch ns); 0 = current time
 DD_TRACE_C_API void dd_span_finish_with_time(dd_span_t* span_handle,
                                               int64_t end_time_ns);
 

binding/c/src/tracer.cpp

@@ -1,11 +1,9 @@
 #include "datadog/c/tracer.h"
 
-#include <datadog/clock.h>
 #include <datadog/hex.h>
 #include <datadog/trace_segment.h>
 #include <datadog/tracer.h>
 
-#include <chrono>
 #include <cstddef>
 #include <cstring>
 #include <string>
@@ -46,19 +44,15 @@ class ContextWriter : public dd::DictWriter {
   }
 };
 
-// Convert a wall-clock timestamp (nanoseconds since UNIX epoch) into a
-// TimePoint. The steady-clock tick is derived from the current offset
-// between system_clock::now() and steady_clock::now() so that duration
-// calculations (which are done in steady-clock space) remain meaningful.
+// tick is derived from the current system/steady offset so that duration
+// (end.tick - start.tick) stays correct across FFI-supplied timestamps.
 dd::TimePoint wall_ns_to_timepoint(int64_t wall_ns) {
   const auto wall = std::chrono::system_clock::time_point(
       std::chrono::duration_cast<std::chrono::system_clock::duration>(
           std::chrono::nanoseconds(wall_ns)));
-  const auto now_wall = std::chrono::system_clock::now();
-  const auto now_tick = std::chrono::steady_clock::now();
-  const auto offset = now_wall - wall;
-  return dd::TimePoint{
-      wall, now_tick - std::chrono::duration_cast<dd::Duration>(offset)};
+  const auto offset = std::chrono::system_clock::now() - wall;
+  return {wall, std::chrono::steady_clock::now() -
+                    std::chrono::duration_cast<dd::Duration>(offset)};
 }
 
 dd::SpanConfig make_span_config(dd_span_options_t options) {

binding/c/test/test_c_binding.cpp

@@ -1,8 +1,6 @@
 #include <datadog/c/tracer.h>
 #include <datadog/tracer.h>
 
-#include <chrono>
-
 #include "mocks/collectors.h"
 #include "null_logger.h"
 #include "test.h"
@@ -116,14 +114,12 @@ TEST_CASE("span create, tag, finish, free", "[c_binding]") {
   CHECK(sd.error == true);
   CHECK(sd.tags.at("error.message") == "something broke");
 
-  const auto start_wall_ns =
-      std::chrono::duration_cast<std::chrono::nanoseconds>(
-          sd.start.wall.time_since_epoch())
-          .count();
-  CHECK(start_wall_ns == start_ns);
-  const auto diff =
-      sd.duration - std::chrono::nanoseconds(end_ns - start_ns);
-  CHECK(std::chrono::abs(diff) < std::chrono::milliseconds(1));
+  CHECK(std::chrono::duration_cast<std::chrono::nanoseconds>(
+            sd.start.wall.time_since_epoch())
+            .count() == start_ns);
+  CHECK(std::chrono::abs(sd.duration -
+                         std::chrono::nanoseconds(end_ns - start_ns)) <
+        std::chrono::milliseconds(1));
 }
 
 TEST_CASE("create span with resource", "[c_binding]") {