AHT is the time from when the phone first rings, until the agent finishes the call and completes there notes and it ready for the next call.
It works something like this.
<pre>Agent is ready for calls.
Phone rings – Timer starts here
Agent answers the call
Agent completes the call
Agent makes notes in the system
Agent makes himself ready for the next call – Times stops here
Agent waits for the next call</pre>