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The form has 2 fields of date fields
field d1
field d2
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)