Carry out the following steps to generate a default Opportunity close date using a field update workflow:

1. Click on the Setup gear icon in the top right-hand corner of the main Home page, as shown in the following screenshot:

2. Click on Setup, as shown in the following screenshot:

3. Navigate to the workflow rule customization setup page as follows:
   Process Automation | Workflow Rules.
4. Click on Continue (if shown).

5. The Continue button is found on the Understanding Workflow page (if this appears), as shown in the following screenshot:

6. Click on New Rule.

We will be presented with the Step 1: Select object page.

7. Select Opportunity and click on Next, as shown in the following screenshot:

We will be presented with the Step 2: Configure Workflow Rule page.

8. Enter Set Opportunity Close Date in the Rule Name field.
9. Optionally, enter a description in the Description field.
10. Select the created option from the Evaluation Criteria options list.
11. In the Rule Criteria section, set the Run this rule if the following option to formula equates to true.
12. Paste or enter the following formula in the formula editor:

NOT(IsClosed)

13. Click on Save & Next, as shown in the following screenshot:

We will be presented with the Step 3: Specify Workflow Actions page.

14. In the Immediate Workflow Actions section, set the Add Workflow Action option to New Field Update, as shown in the following screenshot:

We will be presented with the New Field Update page.

15. Enter Set Opportunity Close Date in the Name field.
16. Accept the default value in the Unique Name field, which is automatically set as Set_Opportunity_Close_Date.
17. Optionally, enter a description in the Description field.
18. Set the Field to Update to Opportunity.
19. Select Close Date.

We will be presented with the Specify New Field Value section.

20. Select the option Use a formula to set the new value.
21. In the Show Formula Editor, paste or enter NOW() + 21.

22. Click on Save, as shown in the following screenshot:

We will be presented with the Step 3: Specify Workflow Actions page.

23. Click on Done, as shown in the following screenshot:

We will be re-directed to the new Workflow Rule, where we can activate the rule.

24. Finally, click on Activate, as shown in the following screenshot:

25. If there is no default workflow user currently set, a user will need to be chosen and you will be alerted with a warning message, as shown in the following screenshot:

26. Click on OK.
27. If there is no default workflow user currently set, we will be re-directed to the Process Automation Settings page where we can set the Default Workflow User, as shown in the following screenshot:

28. Set the Default Workflow User.
29. Click on Save.

We are finally re-directed to the Workflow Rule detail page with the Active checkbox set, as shown in the following screenshot: