Subtract Two Time Stamps

[Fahad Ahmad]

I just started using the script runner for subtracting my custom field and created datetime

I am getting the values of both fields successfuly but i need to subtract them to get the difference. I am unable to subtract since both are in string format. there values are as mentioned below.

2018-12-26T15:12:00.000+0300 custom
2018-12-06T12:59:25.203+0300 issue

I have tried getTime() with them and as Date as well but no success. 

def issueKey = 'SD-9'
def customFieldName = 'customfield_10009'
def issueCreatedKey = 'created'

def result = get("/rest/api/2/issue/${issueKey}")
.header('Content-Type', 'application/json')
.asObject(Map)
if (result.status == 200) {
def custom = result.body.fields[customFieldName]
def issue = result.body.fields[issueCreatedKey]
return custom - issue
} else {
return "Error retrieving issue ${result}"
}

[Fahad Ahmad]

any body can assist please?