Adaptive Card Calculation

We are trying to calculate the number of days between the from and to dates which may be overriden in the Total Days Requested text box in the following JSON:

What is the syntax formula, and how does that get inserted into the JSON Code? 
Thank you

{
    "type": "AdaptiveCard",
    "body": [
        {
            "type": "Container",
            "backgroundImage": {
                "horizontalAlignment": "Center",
                "verticalAlignment": "Center"
            },
            "minHeight": "200px",
            "verticalContentAlignment": "Bottom",
            "items": [
                {
                    "type": "Image",
                    "url": "https://w.png",
                    "altText": "adsf"
                },
                {
                    "type": "ColumnSet",
                    "columns": [
                        {
                            "type": "Column",
                            "width": "400px",
                            "items": [
                                {
                                    "type": "TextBlock",
                                    "text": "PTO Request for: @{body('Parse_JSON')?['EEName']}",
                                    "wrap": true,
                                    "fontType": "Default",
                                    "size": "Large"
                                },
                                {
                                    "type": "TextBlock",
                                    "text": "EEId: @{body('Parse_JSON')?['EEId']}",
                                    "wrap": true,
                                    "id": "EEId"
                                }
                            ],
                            "style": "default"
                        }
                    ]
                }
            ],
            "bleed": true,
            "id": "hero"
        },
        {
            "type": "Container",
            "items": [
                {
                    "type": "TextBlock",
                    "text": "Your current balances",
                    "wrap": true,
                    "weight": "Bolder"
                },
                {
                    "type": "TextBlock",
                    "text": " Available PTO: @{body('Parse_JSON')?['EEPTOAvail']}",
                    "wrap": true,
                    "$when": "${already_planned != ''}"
                },
                {
                    "type": "TextBlock",
                    "text": "🟡 Pending PTO:  @{body('Parse_JSON')?['EEPTOPending']}",
                    "wrap": true
                },
                {
                    "type": "Container",
                    "items": [
                        {
                            "type": "ColumnSet",
                            "columns": [
                                {
                                    "type": "Column",
                                    "width": "stretch",
                                    "items": [
                                        {
                                            "type": "TextBlock",
                                            "text": "🟢  Adjusted PTO Total: @{variables('EEPTotal')}",
                                            "wrap": true,
                                            "size": "Medium",
                                            "weight": "Bolder"
                                        }
                                    ]
                                },
                                {
                                    "type": "Column",
                                    "width": "100px",
                                    "items": [
                                        {
                                            "type": "TextBlock",
                                            "text": " ",
                                            "wrap": true,
                                            "size": "Medium",
                                            "weight": "Bolder"
                                        }
                                    ],
                                    "verticalContentAlignment": "Center",
                                    "horizontalAlignment": "Right"
                                }
                            ],
                            "$data": "${allowances}",
                            "spacing": "Medium"
                        }
                    ],
                    "style": "emphasis",
                    "spacing": "Small"
                },
                {
                    "type": "ActionSet",
                    "actions": [
                        {
                            "type": "Action.ToggleVisibility",
                            "title": "Request PTO now",
                            "targetElements": [
                                "request-pto",
                                "hero",
                                "balances"
                            ]
                        },
                        {
                            "type": "Action.ToggleVisibility",
                            "title": "Cancel",
                            "targetElements": [
                                "request-pto",
                                "hero",
                                "balances"
                            ]
                        }
                    ],
                    "horizontalAlignment": "Left",
                    "id": "toggle"
                }
            ],
            "id": "balances"
        },
        {
            "type": "Container",
            "style": "default",
            "items": [
                {
                    "type": "ColumnSet",
                    "columns": [
                        {
                            "type": "Column",
                            "width": "stretch",
                            "items": [
                                {
                                    "type": "TextBlock",
                                    "text": "From",
                                    "wrap": true,
                                    "size": "Small"
                                },
                                {
                                    "type": "Input.Date",
                                    "spacing": "None",
                                    "id": "from-date"
                                }
                            ]
                        },
                        {
                            "type": "Column",
                            "width": "stretch",
                            "items": [
                                {
                                    "type": "TextBlock",
                                    "text": "To",
                                    "wrap": true,
                                    "size": "Small",
                                    "spacing": "Small"
                                },
                                {
                                    "type": "Input.Date",
                                    "spacing": "None",
                                    "id": "to-date"
                                }
                            ]
                        }
                    ]
                },
                {
                    "type": "Input.Number",
                    "placeholder": "Total Days Requested",
                    "label": "Total Days Requested:",
                    "id": "EETotalDays"
                },
                {
                    "type": "TextBlock",
                    "text": "Type of PTO",
                    "wrap": true,
                    "size": "Small",
                    "spacing": "Small"
                },
                {
                    "type": "Input.ChoiceSet",
                    "choices": [
                        {
                            "title": "Annual Leave",
                            "value": "Choice 1"
                        },
                        {
                            "title": "Parental Leave",
                            "value": "Choice 2"
                        },
                        {
                            "title": "Compassionate Leave",
                            "value": "Choice 3"
                        },
                        {
                            "title": "Jury Duty",
                            "value": "Choice 4"
                        },
                        {
                            "title": "Sick Leave",
                            "value": "Choice 5"
                        }
                    ],
                    "placeholder": "Placeholder text",
                    "spacing": "Small",
                    "id": "type-of-pto",
                    "style": "expanded"
                },
                {
                    "type": "TextBlock",
                    "text": "Comment",
                    "wrap": true,
                    "spacing": "Small",
                    "size": "Small"
                },
                {
                    "type": "Input.Text",
                    "placeholder": "Optional",
                    "spacing": "None",
                    "isMultiline": true,
                    "id": "comment"
                },
                {
                    "type": "ActionSet",
                    "actions": [
                        {
                            "type": "Action.Submit",
                            "title": "Submit PTO request",
                            "id": "_success_Your_PTO_request_was_submitted"
                        },
                        {
                            "type": "Action.ToggleVisibility",
                            "title": "Cancel",
                            "targetElements": [
                                "request-pto",
                                "hero",
                                "balances"
                            ]
                        }
                    ]
                }
            ],
            "id": "request-pto",
            "isVisible": false
        }
    ],
    "$schema": "http://adaptivecards.io/schemas/adaptive-card.json",
    "version": "1.4"
}