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The following screenshot contains sample data that must be copied to Due Date:

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Here, next due date is a custom Date field whose value is set to 18/Dec/19. Use a post function to copy this value on transition.

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1. Log in as an administrator in your Jira instance.

2. Select Cog menu () > Issues.
      

3. Select Issues from the top menu bar and click Custom Fields.
   
   

4. Search for the next due date custom field, select Cog menu (Image RemovedImage Added) > Configure.
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5. Anchor
   customFieldID customFieldID

   Custom field ID is shown as customFieldId from in the URL. Note just this parameter and value pair as shown:
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Use the following instructions to update the post function in a workflow and copy the custom field value into the Due date field:

1. Log in as an administrator into your Jira instance.
2. Select Projects from menu bar and click the project name. If the project is not visible from the drop-down, click View all projects, and, select the relevant project.
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3. Click Project settings on the Project page to administer the project.
4. Go to the Workflows section and edit the relevant workflow scheme.
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5.  Click the transition with the post function to be updated. As an example, update the Done post function.
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6. Click Post Functions, and then, click the pencil icon to edit the function.
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7. Click Detailed Fields. Enter the substitution variable, %original_customfield_10300%, in the Due date field. Note that this custom field ID was copied from the URL from step 5 in the previous section.
   

8. Publish the Workflow and perform the Done Transition in the Jira issue. 

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