Under the Timesheets module (sidebar menu), there are several ways to manually add time:
First, select the desired user followed by its appropriate projects and tasks assigned to them (left panel). Then add time as either interval or duration, together with any notes for yourself or the client (right panel).
NOTE: You can also customize the time increments as you drag on the timeline.
To do so, click on Timesheet Settings (gear icon) → Snap Interval → Choose between 5/10/15/30 minute increments.
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Timesheet Views
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