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You can schedule a job to assign issues to alternative assignees when the original assignee is unavailable.
Shift Management
Global Shifts
You can specify users' working hours in the Shift Management Panel!
Manage Apps > Time Off Management & Auto Assignment > Shift Management > Create Shift
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When you define the shifts, you can use the Assign By Shift post-function. In the below example, Alana and Mitch were selected as the assignees. When when an issue is created, the assignee will selected according to the shifts.
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RP-29 was created on Wednesday, August 16, at 4:58 PM. According to the defined shifts above, it is assigned to Miched Mitch rather than Alana.
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Project Shifts
You can specify users' working hours per Jira project.
Project Settings > Time Off Management & Auto Assignment > Shifts > Create Shift