Doing work estimates on issue or sub-task level and setting the remaining estimate when logging work is a JIRA feature, and ictime also uses this data. As regards doing estimates and displaying estimates, time logged and remaining estimate on task level, ictime does not add any functionality (but offers the possibility to take away some options, see below). For documentation on JIRA level, please refer to http://confluence.atlassian.com/display/JIRA/Logging+Work+on+an+Issue.
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Set Remaining Estimate (When Logging Work)
Whenever logging work in ictime - via the log work screen or as part of a workflow transition or any other screen - you have the option to define how the remaining estimate should be set. Depending on the screen, JIRA your configuration of ictime regarding this feature (see General Configuration), ictime offers a flexible selection to automatically calculate, leave unchanged, reduce by or manually set the new remaining estimate. If there has been no original estimate yet, you can also decide to leave it blank. This is exactly the same functionality JIRA would offer (see https://confluence.atlassian.com/display/JIRA/Logging+Work+on+an+Issue#LoggingWorkonanIssue-logworkviewingLoggingworkwhenviewinganissue).
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Depending on your point of view, this makes sense e.g. if you decide to adapt the remaining value as you have noticed that you will need more or less time compared to the original estimate, but you don't want to change the original estimate. At this moment, values for estimate, time logged and remaining estimate do no longer "fit" together, but it is intended that they don't fit. It might be perfectly okay that your values show that the original estimate had been 24 hours, you have already worked 12 hours, and the new remaining estimate is still 20 hours. Maybe in such cases you are used to simply change the planned original estimate, but of course this is not always a good practise, because it "hides" the fact that the original estimate was not fitting.
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You can also configure ictime in a way that users do not have different options to set the remaining estimate, but has no choice and ictime will simply calculate the remaining estimate automatically (see General Configuration).
Set Remaining Estimate (When Deleting Logging Work)
You also have similar options when deleting a work log (depending on your ictime configuration, see General Configuration).
Use ictime Planning Sheet
Instead of planning time by doing work estimates when creating or editing an issue, you can also use the central planning sheet on project level to plan time for issues. Please refer to Planning Sheet.