You can add a new or existing issue to calendar. Simply select a calendar date and time, and click on it. You can also stretch the duration of the issue.
Changing the duration of an issue in the calendar will automatically update the "Original Estimate" field.
The "Original Estimate" is calculated based on the length of the working day (in hours) and the working week (days) that are in your system settings JIRA (tab "Time Tracking").