SharePoint Online JSON script to custom format a person column with a mailto link. This example shows how to convert a column date content to a string. The operators toString() or toLocaleString() could also be used, depending on the desired format output.

SPO-Column-Formating-Example08.json

{
    "$schema": "http://columnformatting.sharepointpnp.com/columnFormattingSchema.json",
    "debugMode": true,
    "elmType": "div",
    "children": [
        {
            "elmType": "span",
            "style": {
                "padding-right": "8px"
            },
            "txtContent": "@currentField.title"
        },
        {
            "elmType": "a",
            "attributes": {
                "iconName": "EditMail",
                "class": "sp-field-quickActions",
                "href": {
                    "operator": "+",
                    "operands": [
                        "mailto:",
                        "@currentField.email",
                        "?subject=Check task status - ",
                        "[$Title]",
                        "&body=Please update your task status\r\n---\r\n",
                        "[$Title]",
                        "This task is due on: ",
                        {
                            "operator": "toLocaleDateString()",
                            "operands": [
                                "[$Date]"
                            ]                            
                        },
                        "\r\nThanks."
                    ]
                }
            }
        }
    ]
}