Timers
Timers can be used to keep track of how long you are spending on certain tasks. When a timer has been stopped it will automatically create a time log entry.
There are several ways to start a timer:
Header Bar Timer
Click the play button on the timer in the header bar, as shown below:

Time Form
You can start a timer from the time form:

Work Item Slider
You can start a timer when viewing a work item slider:

Work Items Table View
You can start a timer from the work items table view, if you have configured you table to display the timer:

Timer Dropdown Menu
If you have already logged time against something during the current week, then you can click on the header bar timer dropdown to see those items. From there you can restart another timer. You can also navigate back to previous weeks to find items that you worked on previously:
