What is Builtin_DateNow from node.js profile result?

First, I start my Node.js program using node --prof command

Then, I use ab tool to test the perfomance.

Finally, I use node --prof-process to analyze the profile log.

According to the result, the bottleneck should be:

 [Bottom up (heavy) profile]:
  Note: percentage shows a share of a particular caller in the total
  amount of its parent calls.
  Callers occupying less than 2.0% are not shown.

   ticks parent  name
  269973   40.0%  write
  263992   97.8%    v8::internal::Builtin_DateNow(int, v8::internal::Object**, v8::internal::Isolate*)
  200802   76.1%      LazyCompile: *Document.$__buildDoc /home/vagrant/Fundebug/api-server/node_modules/.4.9.6@mongoose/lib/document.js:159:42
  200433   99.8%        LazyCompile: *Document /home/vagrant/Fundebug/api-server/node_modules/.4.9.6@mongoose/lib/document.js:39:18
  200433  100.0%          LazyCompile: *Model /home/vagrant/Fundebug/api-server/node_modules/.4.9.6@mongoose/lib/model.js:50:15
  200433  100.0%            LazyCompile: *model /home/vagrant/Fundebug/api-server/node_modules/.4.9.6@mongoose/lib/model.js:3596:27
  63117   23.9%      Handler: populated
  63117  100.0%        LazyCompile: *Document /home/vagrant/Fundebug/api-server/node_modules/.4.9.6@mongoose/lib/document.js:39:18
  63117  100.0%          LazyCompile: *Model /home/vagrant/Fundebug/api-server/node_modules/.4.9.6@mongoose/lib/model.js:50:15
  63117  100.0%            LazyCompile: *model /home/vagrant/Fundebug/api-server/node_modules/.4.9.6@mongoose/lib/model.js:3596:27

However, what is Builtin_DateNow? How can I optimize the performance according to this result?

via KiwenLau