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When you manually create a worklog, you have the option to add Remaining Time. This allows you to show an estimate of how much time will be needed to resolve the issue. Whenever work is logged on the issue, the remaining time will be recalculated.
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The Remaining Time estimate can be manually changed at any time. If it is changed, the original total estimate will also be shown on the worklog.
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Note that you can also click on the … menu select Edit to open the worklog dialog and adjust the Remaining Timewhen creating new worklogs, or by editing an existing worklog.