Check load time of div

Check load time of div

I have a div which downloads content from an external resource and then
displays it.
How can I determine how long it takes to load this div ?
I could use javascript to output the current timestamp before the div and
after and then use the difference in time to discover the load time :
pseudo code :
<script>
console.log(currentTime)
</script>
<div>
load external resource
</div>
<script>
console.log(currentTime)
</script>
Should I use a tool like dynatrace ?