As per MDN,

CodePudding user response：

The browser queries the operating system (whichever one you are using) to determine the user's timezone. For instance on the Mac I'm typing this answer on:

#if V8_OS_DARWIN
#include <mach/mach.h>
#include <mach/mach_time.h>
#include <pthread.h>
#endif

#if V8_OS_DARWIN
int64_t ComputeThreadTicks() {
  mach_msg_type_number_t thread_info_count = THREAD_BASIC_INFO_COUNT;
  thread_basic_info_data_t thread_info_data;
  kern_return_t kr = thread_info(
      pthread_mach_thread_np(pthread_self()),
      THREAD_BASIC_INFO,
      reinterpret_cast<thread_info_t>(&thread_info_data),
      &thread_info_count);
  CHECK_EQ(kr, KERN_SUCCESS);

  // We can add the seconds into a {int64_t} without overflow.
  CHECK_LE(thread_info_data.user_time.seconds,
           std::numeric_limits<int64_t>::max() -
               thread_info_data.system_time.seconds);
  int64_t seconds =
      thread_info_data.user_time.seconds   thread_info_data.system_time.seconds;
  // Multiplying the seconds by {kMicrosecondsPerSecond}, and adding something
  // in [0, 2 * kMicrosecondsPerSecond) must result in a valid {int64_t}.
  static constexpr int64_t kSecondsLimit =
      (std::numeric_limits<int64_t>::max() /
       v8::base::Time::kMicrosecondsPerSecond) -
      2;
  CHECK_GT(kSecondsLimit, seconds);
  int64_t micros = seconds * v8::base::Time::kMicrosecondsPerSecond;
  micros  = (thread_info_data.user_time.microseconds  
             thread_info_data.system_time.microseconds);
  return micros;
}

You can see here where chromium queries the (again OS-specific) system time.

Source

If you look at the linked file in the codebase you can see where it has ifdefs for all the various supported OS flavors: win32, POSIX, Fuschia, etc.