fix type convert

src/core/lib/support/time_posix.c

@@ -127,7 +127,7 @@ static gpr_timespec now_impl(gpr_clock_type clock) {
       now.tv_nsec = now_tv.tv_usec * 1000;
       break;
     case GPR_CLOCK_MONOTONIC:
-      now_dbl = (mach_absolute_time() - g_time_start) * g_time_scale;
+      now_dbl = ((double)(mach_absolute_time() - g_time_start)) * g_time_scale;
       now.tv_sec = (int64_t)(now_dbl * 1e-9);
       now.tv_nsec = (int32_t)(now_dbl - ((double)now.tv_sec) * 1e9);
       break;