The form has 2 fields of date fields
This form sets the value for field d2 to 15 days AFTER the d1
How the form looks like in the editor

and the rule that sets the date in particular
With "values to set" parameter set to
d2=([entry.d1.timestamp] +1296000000)
where 1296000000 is really 15*86400000 (15 days multiplied by milliseconds per day)
Storage format for the solution
Storage format
<ac:structured-macro ac:macro-id="ddfa8bbd-a3e8-41a3-8df2-049cefe28d71" ac:name="confiform" ac:schema-version="1">
<ac:parameter ac:name="formName">f1</ac:parameter>
<ac:rich-text-body>
<ac:structured-macro ac:macro-id="32c99bb3-be63-434c-8fd7-52b5055473a9" ac:name="confiform-entry-register" ac:schema-version="1">
<ac:parameter ac:name="embedded">true</ac:parameter>
<ac:rich-text-body>
<p> </p>
</ac:rich-text-body>
</ac:structured-macro>
<p>
<ac:structured-macro ac:macro-id="b26e98b6-3cb4-4f52-9395-e60f5d349e80" ac:name="confiform-field-definition" ac:schema-version="1">
<ac:parameter ac:name="fieldName">d1</ac:parameter>
<ac:parameter ac:name="fieldLabel">d1</ac:parameter>
<ac:parameter ac:name="type">date</ac:parameter>
</ac:structured-macro>
</p>
<p>
<ac:structured-macro ac:macro-id="00cd10f9-bd01-4566-bc1f-9098f364fa5b" ac:name="confiform-field-definition" ac:schema-version="1">
<ac:parameter ac:name="fieldName">d2</ac:parameter>
<ac:parameter ac:name="fieldLabel">d2</ac:parameter>
<ac:parameter ac:name="type">date</ac:parameter>
</ac:structured-macro>
</p>
<p>
<ac:structured-macro ac:macro-id="5f453ca1-8d2d-472a-b4aa-a4c9a3b45339" ac:name="confiform-field-definition-rules" ac:schema-version="1">
<ac:parameter ac:name="condition">d1:[empty]</ac:parameter>
<ac:parameter ac:name="values">d1=[now]</ac:parameter>
<ac:parameter ac:name="action">Set value</ac:parameter>
</ac:structured-macro>
</p>
<p>
<ac:structured-macro ac:macro-id="8b95d332-e5c6-46ad-8b9a-80c5c8f085ad" ac:name="confiform-field-definition-rules" ac:schema-version="1">
<ac:parameter ac:name="condition">!d1:[empty]</ac:parameter>
<ac:parameter ac:name="values">d2=([entry.d1.timestamp] +1296000000)</ac:parameter>
<ac:parameter ac:name="action">Set value</ac:parameter>
<ac:parameter ac:name="actionFieldName">d1</ac:parameter>
</ac:structured-macro>
</p>
<p> </p>
</ac:rich-text-body>
</ac:structured-macro>