The timer button on the task can be switched on to start tracking the hours you spend on this particular issue. Unless switched off, the timer will continue to run, and will show up on your dashboard as a reminder.
Use the timer dropdown to view options for time tracking. You can either switch on the timer, as explained above or choose the manual option to punch in time spent on the task as well as the date. You can also add a time range (from 9 AM to 10 PM).
Time tracked will show up on the timelog section. You can open specific entries and edit them for more accuracy.
You can also toggle the billable hours button to ensure invoicing becomes easy.