CRM2013 SP1 has added a new control called timer control. Once the timer control is setup and added to the form, it starts ticking based on the various setup criteria. Getting the current timer value using Javascript is easy. Just use updateTimeDifferenceInSeconds method, which will return the current ticks in seconds.
Dreaming in CRM & Power Platform 
The above javascript does not work in Dynamics 365.
Any clues on how we can read the value in D364?
You are correct. This does not work in Dynamics 365. This is the danger of using undocumented code I guess. I haven’t been able to find any workaround yet.