Getting the milliseconds between 2 TestSteps

I am trying to check the performance of a deploy API.

I am using these steps to get the time it takes to deploy. 

-Start Time ([Groovy Script] This sets a custom property with the current time in this format "HH:mm:ss.SSS")

-Deploy ([API] This only start the job, we need another step to make sure the job is done and done right)

-Transfer jobID (Standard property transfer)

-Query Job Status ([API] This gets the status of a job from the jobID)

-Check Job Status ([Groovy Script] a script that goto "Query Job Status" if job is not complete)

-End Time (This sets a custom property with the current time in this format "HH:mm:ss.SSS" and adds 1 to a counter)

-Loop condition (If counter is not equal to predetermined amount, goto TestStep "StartTime")

How do I calculate the milliseconds between the 2 custom properties "StartTime" and "EndTime"?

Or

How do I get the time between 2 TestSteps?

Whichever makes more sense to use.

This is how I create the time stamp in the custom properties:

String strCurrentDate = new Date().format("HH:mm:ss.SSS")
startTlime = strCurrentDate
testRunner.testCase.setPropertyValue("StartTime", startTime)

This is my last attempt at getting the difference between the 2 values

def startTime = testRunner.testCase.getPropertyValue( 'StartTime' )
def endTime = testRunner.testCase.getPropertyValue( 'EndTime' )
Date ParsedStartTime = Date.parse("HH:mm:ss.SSS", startTime)
Date ParsedEndTime = Date.parse("HH:mm:ss.SSS", endTime)
Date rateDuration = Date.(ParsedEndTime - ParsedStartTime)

I've tried many ways to manipulate the data like String, Date, .format(), .parse(), alone and combined. I am not able to make it work

Any suggestions?