How do I autofill a Date field based on 2 other field results?

[Liam Denvir]

I want to automatically populate the Target start date of an issue based on the responses in two other fields (SizeOfMatter & TargetEndDate). I'm not sure how to annotate this in confiforms & have tried automating this on issue creation in Jira, but while the rule completes, the Target start date field remains 'None'.

Fields with set responses

"Description": "*Time required* : [entry.SizeOfMatter]", - has 4 options, with IDs 1, 2, 3, 4. 

"customfield_2":"[entry.TargetEndDate.jiraDate]", - this is selected by the submitter.

Rule to be created
IF [entry.SizeOfMatter] = 1, then "customfield_1" : "[customfield_2.minusDay(14)]",

Is this something that's possible to do through confiforms?

[Alex Medved _ConfiForms_]

Something like this may be

#if(${SizeOfMatter} == "1")
 "customfield_1": "[entry.TargetEndDate.add(-1209600000).jiraDate]",
#end

(1,209,600,000 is the number of milliseconds in 14 days)