Could anyone recommend a method of tracking call duration within a call form?

I understand there is "call duration" within the Calls module. Might be good enough if we were using that module, but our current modules are custom built with Studio. We were hoping for something like a timer within the form (start/stop buttons), system-tracked as opposed to agent-entered. Perhaps an add-on project? Unfortunately our time to implement the call center's tracking precludes implementing CTI or some more preferable integration with the phone system.

Any help or suggestions would be most appreciated.