If the duration of your time entry doesn’t reflect the time spent on the task check the timesheet’s settings - click on the three-dot icon above the green Start timer button and check the option called Synchronise timeframe with duration when editing time entry.
If it is turned on, the duration will reflect the start and end time of entry. It helps to automatically match timeframes and the duration time of your entry while modifying entries.
If it is turned off timeframes of entry and duration time will not match if you edit one of its values. It helps to easily check and edit the correctness of the Timesheet, but each value (start, end, duration) should be manually inputted.
Changing the "Synchronise timeframe with duration when editing time entry" setting won't affect the already added time. It will be applied only to future time entries.
Here you can find more examples of how exactly this feature works.