How to set the transaction time programatically using Truclient?

I wish to capture the execution time of a backend processing job and show this as a transaction in the Analaysis report. We have a UI page that display the backend job start time and completed time. I can probably write some javascript code to extract information from this UI page and evaluate the job execution time, then output the execution time in the vugen log. But I wish to also see this as a transaction in the Analysis report. Is there a way this can be done? 

It would be great if we can programatically set the transaction time.

Thanks!
Jason