v 4.0.x Start/Stop timers
Timer functionality can be used for an automatic calculation of the spent time.
Timer dialog
Timer dialog can be launched from any Jira screen through My Work -> Timer menu. Here you can start/stop timers and create log records from the timer records.
The dialog shows last 5 timers that belong to current user. A timer record can be in "Active", "Paused" and "Off" statuses. "Start timer", "Pause timer" and "Clear timer" buttons transfer a timer into an appropriate status. Clicking "Report time" button shows "Log Work" dialog populated with issue key, description and time from the source timer.
When a new timer is started while there is an existing active timer, the active timer is paused. Once you log work based on an active timer, the source timer stops.
Timer panel
 "Timer" panel is shown at the right side of the issue screen. This is a convenient way to start/stop a timer for an issue you are currently working on.